SC4503 [SEMTECH]
1.3MHz Step-Up Switching Regulator with 1.4A Switch; 1.3MHz升压型开关稳压器与1.4A开关
| 型号： | SC4503 |
| 厂家： | 
SEMTECH CORPORATION |
| 描述： | 1.3MHz Step-Up Switching Regulator with 1.4A Switch |
| 文件： | 总22页 (文件大小：1176K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
SC4503
1.3MHz Step-Up Switching
Regulator with 1.4A Switch
POWER MANAGEMENT
Features
Description
The SC4503 is a 1.3MHz current-mode step-up switch- Low Saturation Voltage Switch: 260mV at 1.4A
ing regulator with an integrated 1.4A power transistor. 1.3MHz Constant Switching Frequency
Its high switching frequency allows the use of tiny sur- Peak Current-mode Control
face-mount external passive components. The SC4503 Internal Compensation
features a combined shutdown and soft-start pin. The Programmable Soft-Start
optional soft-start function eliminates high input current Input Voltage Range From 2.5V to 20V
and output overshoot during start-up. The internal com- Output Voltage up to 27V
pensation network accommodates a wide range of volt- Uses Small Inductors and Ceramic Capacitors
age conversion ratios. The internal switch is rated at 34V Low Shutdown Current (< 1μA)
making the device suitable for high voltage applications Low Profile 5-Lead TSOT-23 and 8-Lead 2X2mm
such as Boost and SEPIC.
MLPD-W packages
Fully WEEE and RohS compliant
The SC4503 is available in low-profile 5-lead TSOT-23 and
8-lead 2X2mm MLPD-W packages. The SC4503’s low
shutdown current (< 1μA), high frequency operation and
small size make it suitable for portable applications.
Applications
Local DC-DC Converters
TFT Bias Supplies
XDSL Power Supplies
Medical Equipment
Digital Cameras
Portable Devices
White LED Drivers
Typical Application Circuit
D1
L1
Efficiency vs Load Current
VIN
5V
VOUT
95
90
85
80
75
70
65
60
55
50
12V, 0.5A
4.7µH
1.3MHz
10BQ015
5
1
C4
15pF
R1
IN
SW
432k
SC4503
C1
1µF
C2
4.7µF
4
3
ON
OFF
SHDN/SS
GND
FB
R2
49.9k
2
VOUT = 12V
C1: Murata GRM188R61A105K
C2: Murata GRM21BR61C475K
L1: Sumida CDC5D23B-4R7
0.001
0.010
0.100
1.000
Load Current (A)
Figure 1(b). Efficiency of the 5V to 12V Boost Converter
Figure 1(a). 5V to 12V Boost Converter
May 4, 2007
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SC4503
POWER MANAGEMENT
Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the
Electrical Characteristics section is not recommended.
Parameter
Symbol
VIN
Maximum
-0.3 to 20
-0.3 to 34
-0.3 to VIN +0.3
-0.3 to VIN +1
191*
Units
Supply Voltage
SW Voltage
VSW
VFB
V
FB Voltages
SHDN/SS Voltage
VSHDN
θ JA
Thermal Resistance Junction to Ambient (TSOT - 23)
Thermal Resistance Junction to Ambient (2X2 mm MLPD-W)
Maximum Junction Temperature
Storage Temperature Range
Lead Temperature (Soldering)10 sec (TSOT - 23)
Peak IR Reflow Temperature (2X2mm MLPD-W)
ESD Rating (Human Body Model)
°C/W
°C/W
θ JA
78*
TJ
150
TSTG
TLEAD
TIR
-65 to +150
260
°C
V
260
ESD
2000
*Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad as per JESD51 standards.
Electrical Characteristics
Unless specified: VIN = VSHDN/SS = 3V, -40°C < TA = TJ < 85°C
Parameter
Conditions
Min
1.225
1.15
Typ
Max
2.5
Units
Under-Voltage Lockout Threshold
Maximum Operating Voltage
Feedback Voltage
2.2
20
V
1.250
0.02
-25
1.275
Feedback Line Voltage Regulation
FB Pin Bias Current
2.5V < VIN < 20V
%/V
nA
-50
1.55
0
Switching Frequency
1.30
MHz
Minimum Duty Cycle
%
Maximum Duty Cycle
86
90
1.9
Switch Current Limit
1.4
2.5
430
1
A
Switch Saturation Voltage
Switch Leakage Current
VIN Quiescent Supply Current
ISW = 1.4A
VSW = 5V
260
0.01
0.8
mV
µA
mA
µA
VSHDN/SS = 2V, VFB = 1.5V (not switching)
1.1
1
VIN Supply Current in Shutdown
VSHDN/SS = 0
0.01
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SC4503
POWER MANAGEMENT
Electrical Characteristics (Cont.)
Unless specified: VIN = VSHDN/SS = 3V, -40°C < TA = TJ < 85°C
Parameter
Conditions
Min
Typ
Max
Units
SHDN/SS Switching Threshold
Shutdown Input High Voltage
Shutdown Input Low Voltage
VFB = 0V
1.4
V
2
V
0.4
50
VSHDN/SS = 2V
VSHDN/SS = 1.8V
VSHDN/SS = 0V
22
20
SHDN/SS Pin Bias Current
45
µA
°C
0.1
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
155
10
Pin Configuration - TSOT - 23
Ordering Information
Device(1,2)
Top Mark
Package
TSOT-23
Top View
SC4503TSKTRT
BH00
SW
GND
FB
IN
1
2
3
5
4
SC4503EVB
Notes:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Available in lead-free package only. Device is WEEE and
RoHS compliant.
Evaluation Board
SHDN/SS
5-LEAD TSOT-23
Pin Descriptions - TSOT -23
Pin
1
Pin Name Pin Functions
Collector of the internal power transistor. Connect to the boost inductor and the freewheeling
diode. The maximum switching voltage spike at this pin should be limited to 34V.
SW
2
GND
Ground. Tie to ground plane.
The inverting input of the error amplifier. Tie to an external resistive divider to set the output volt-
age.
3
FB
Shutdown and Soft-start Pin. Pulling this pin below 0.4 shuts down the converter. Applying more
than 2V at this pin enables the SC4503. An external resistor and an external capacitor con-
nected to this pin soft-start the switching regulator. The SC4503 will try to pull the SHDN/SS pin
4
5
SHDN/SS below its 1.4V switching threshold regardless of the external circuit attached to the pin if VIN
is below the under-voltage lockout threshold. Tie this pin through an optional resistor to IN or
to the output of a controlling logic gate if soft-start is not used. See Applications Information for
more details.
Power Supply Pin. Bypassed with capacitor close to the pin.
IN
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SC4503
POWER MANAGEMENT
Pin Configuration - 2mm X 2mm MLPD
Ordering Information
Top View
Device(1,2)
Top Mark
Package
2mmX2mm
MLPD-W
8
7
1
2
NC
SW
SW
SC4503WLTRT
E00
GND
GND
FB
SC4503_MLPD EVB
Notes:
Evaluation Board
6
5
IN
3
4
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Available in lead-free package only. Device is WEEE and
RoHS compliant.
SHDN/SS
8-LEAD 2X2mm MLPD-W
Pin Descriptions - 2X2mm MLPD-W
Pin
1,2
3
Pin Name
Pin Functions
Collector of the internal power transistor. Connect to the boost inductor and the free-
wheeling diode. The maximum switching voltage spike at this pin should be limited to
34V.
SW
IN
Power Supply Pin. Bypassed with capacitor close to the pin.
Shutdown and Soft-start Pin. Pulling this pin below 0.4 shuts down the converter. Apply-
ing more than 2V at this pin enables the SC4503. An external resistor and an external
capacitor connected to this pin soft-start the switching regulator. The SC4503 will try
to pull the SHDN/SS pin below its 1.4V switching threshold regardless of the external
circuit attached to the pin if VIN is below the under-voltage lockout threshold. Tie this pin
through an optional resistor to IN or to the output of a controlling logic gate if soft-start is
not used. See Applications Information for more details.
4
SHDN/SS
The inverting input of the error amplifier. Tie to an external resistive divider to set the
output voltage.
5
FB
6,7
8
GND
N.C.
Ground. Tie to ground plane.
No Connection.
EDP
Solder to the ground plane of the PCB.
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SC4503
POWER MANAGEMENT
Block Diagram
IN
5
SW
1
+
1V
-
Z1
REF NOT READY
Q2
SHDN/SS
VOLTAGE
4
T > 155°C
THERMAL
J
REFERENCE
SHUTDOWN
CLK
1.25V
+
-
R
S
FB
EA
-
Q3
Q
2
PWM
+
+
D1
I-LIMIT
ILIM
-
Q1
R
SENSE
+
ISEN
Σ
-
+
OSCILLATOR
SLOPE COMP
2
GND
Figure 2. SC4503 Block Diagram
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SC4503
POWER MANAGEMENT
Typical Characteristics
Switching Frequency
vs Temperature
FB Voltage vs Temperature
1.30
1.25
1.20
1.15
1.10
1.5
1.4
1.3
1.2
1.1
1.0
-50 -25
0
25 50 75 100 125
-50 -25
-50 -25
-50 -25
0
25 50 75 100 125
Temperature (°C)
Temperature (°C)
Switch Current Limit
vs Temperature
VIN Under-voltage Lockout
Threshold vs Temperature
2.0
1.8
1.6
1.4
1.2
1.0
2.6
2.4
2.2
2.0
1.8
1.6
VSHDN/SS = 3V
-50 -25
0
25 50 75 100 125
0
25 50 75 100 125
Temperature (°C)
Temperature (°C)
V
IN Quiescent Current
Switch Saturation Voltage
vs Switch Current
vs Temperature
400
300
200
100
0
0.80
0.75
0.70
0.65
0.60
125°C
25°C
-40°C
VFB = 1.5V
0
25 50 75 100 125
0.0
0.5
1.0
1.5
2.0
Temperature (°C)
Switch Current (A)
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SC4503
POWER MANAGEMENT
Typical Characteristics (Cont.)
Shutdown Pin Current
vs Shutdown Pin Voltage
Shutdown Pin Current
vs Shutdown Pin Voltage
70
50
40
30
20
10
0
-40°C
60
50
-40°C
25°C
25°C
40
30
20
85°C
10
85°C
2.0
0
0.0
0.5
1.0
1.5
2.5
3.0
0
5
10
15
20
Shutdown Pin Voltage (V)
Shutdown Pin Voltage (V)
VIN Quiescent Current
Shutdown Pin
vs Shutdown Pin Voltage
Thresholds vs Temperature
1000
800
600
400
200
0
1.5
1.0
0.5
0.0
VIN = 3V
VFB = 1.5V
Switching
125°C
25°C
Shutting Down To IIN < 1 A
µ
-40°C
-50 -25
0
25 50 75 100 125
0.0
0.5
1.0
1.5
2.0
Temperature (°C)
Shutdown Pin Voltage (V)
Switch Current Limit
Switch Current Limit
vs Shutdown Pin Voltage
vs Shutdown Pin Voltage
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
D = 50%
D = 80%
-40°C
-40°C
25°C
85°C
25°C
85°C
1.2
1.4
1.6
1.8
2.0
1.2
1.4
1.6
1.8
2.0
Shutdown Pin Voltage (V)
Shutdown Pin Voltage (V)
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SC4503
POWER MANAGEMENT
Applications Information
Operation
clamped by D1 and Q1, follows the voltage at the SHDN SS
pin. The input inductor current, which is in turn controlled
by the error amplifier output, also ramps up gradually.
Soft-starting the SC4503 in this manner eliminates high
input current and output overshoot. Under fault condition
(VIN < 2.2V or over-temperature), the soft-start capacitor is
discharged to 1V. When the fault condition disappears, the
converter again undergoes soft-start.
The SC4503 is a 1.3MHz peak current-mode step-up
switching regulator with an integrated 1.4A (minimum)
power transistor. Referring to the block diagram, Figure
2, the clock CLK resets the latch and blanks the power
transistor Q3 conduction. Q3 is switched on at the trailing
edge of the clock.
Switch current is sensed with an integrated sense resistor.
The sensed current is summed with the slope-compensat-
ing ramp and fed into the modulating ramp input of the
PWM comparator. The latch is set and Q3 conduction is
terminated when the modulating ramp intersects the error
amplifier (EA) output. If the switch current exceeds 1.9A (the
typical current-limit), then the current-limit comparator ILIM
will set the latch and turn off Q3. Due to separate pulse-
width modulating and current limiting paths, cycle-by-cycle
current limiting is not affected by slope compensation.
Setting the Output Voltage
An external resistive divider R1 and R2 with its center tap
tied to the FB pin (Figure 3) sets the output voltage.
9287
§
¨
©
·
¸
¹
5ꢀ = 5ꢁ
−ꢀ
(1)
ꢀꢃꢁꢂ9
VOUT
SC4503
R1
25nA
The current-mode switching regulator is a dual-loop feed-
back control system. In the inner current loop the EA output
controls the peak inductor current. In the outer loop, the
error amplifier regulates the output voltage. The double
reactive poles of the output LC filter are reduced to a single
real pole by the inner current loop, allowing the internal loop
compensation network to accommodate a wide range of
input and output voltages.
3
FB
R2
Figure 3. R1- R2 Divider Sets the Output Voltage
The input bias current of the error amplifier will introduce
an error of:
Applying 0.9V at the SHDN SS pin enables the voltage refer-
ence. The signal “REF NOT READY” does not go low until
VIN exceeds its under-voltage lockout threshold (typically
2.2V). Assume that an external resistor is placed between
ꢁꢂQ$ •
5ꢄ°«5ꢁ
•ꢄꢅꢅ
∆9287
= −
ꢀ
(2)
9287
ꢄꢃꢁꢂ9
SHDN SS
the IN and the
pins during startup. The voltage
SHDN SS
reference is enabled when the
voltage rises to
The percentage error of a VOUT = 5V converter with R1 =
100kΩ and R2 = 301kΩ is
0.9V. Before VIN reaches 2.2V, “REF NOT READY” is high.
Q2 turns on and the Zener diode Z1 loosely regulates the
SHDN SS
voltage to 1V (above the reference enabling volt-
ꢅꢂQ$ •
ꢁꢄꢄN°«ꢆꢄꢁN
•ꢁꢄꢄ
∆9287
= −
= −ꢄꢃꢁꢂꢀ
age). The optional external resistor limits the current drawn
during under-voltage lockout.
9287
ꢁꢃꢅꢂ9
This error is much less than the ratio tolerance resulting
from the use of 1% resistors in the divider string.
When VIN exceeds 2.2V, “REF NOT READY” goes low. Q2 turns
SHDN SS
off, releasing
. If an external capacitor is connected
SHDN SS
SHDN SS
voltage
from the
pin to the ground, the
will ramp up slowly. The error amplifier output, which is
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
where ILIM is the switch current limit.
Duty Cycle
It is worth noting that IOUTMAX is directly proportional to the
9
The duty cycle D of a boost converter in continuous-conduc-
tion mode (CCM) is:
,1
ratio
and that switching losses are neglected in its
9287
derivation. Equation (4) therefore over-estimates the
maximum output current, however it is a useful first-order
approximation.
9
,1
ꢀ −
9287 + 9'
9&(6$7
' =
(3)
ꢀ −
9287 + 9'
Using VCESAT = 0.3V, VD = 0.5V and ILIM =1.4A in (3) and (4),
the maximum output current for three VIN and VOUT combi-
nations are tabulated (Table 1).
where VCESAT is the switch saturation voltage and VD is volt-
age drop across the rectifying diode.
Maximum Output Current
VIN (V)
VOUT (V)
D
IOUT (A)
3.3
3.3
5
12
5
0.754
0.423
0.615
0.34
0.80
0.53
In a boost switching regulator the inductor is connected
to the input. The inductor DC current is the input current.
When the power switch is turned on, the inductor current
flows into the switch. When the power switch is off, the
inductor current flows through the rectifying diode to the
output. The output current is the average diode current.
The diode current waveform is trapezoidal with pulse width
(1 – D)T (see Figure 4). The output current available from
12
Table 1. Calculated Maximum Output Currents
Maximum Duty-Cycle Limitation
The power transistor in the SC4503 is turned off every
switching period for 80ns. This minimum off time limits the
maximum duty cycle of the regulator. A boost converter with
9287
I
IN
Inductor
Current
Switch Current
high
ratio requires long switch on time and high duty
ON
OFF
ON
9
,1
0
Diode Current
cycle. If the required duty cycle is higher than the attain-
able maximum, then the converter will operate in dropout.
(Dropout is a condition in which the regulator cannot attain
its set output voltage below current limit.)
DT
(1-D)T
ON
I
OUT
ON
OFF
OFF
ON
0
Note: dropout can occur when operating at low input volt-
ages (<3V) and with off times approaching 100ns. Shorten
the PCB trace between the power source and the device
input pin, as line drop may be a significant percentage of
the input voltage. A regulator in dropout may appear as
if it is in current limit. The cycle-by-cycle current limit of
the SC4503 is duty-cycle and input voltage invariant and
should be at least 1.4A. If the converter output is below
its set value and switch current limit is not reached (1.4A),
then the converter is likely in dropout.
Figure 4. Current Waveforms in a Boost Converter
a boost converter therefore depends on the converter oper-
ating duty cycle. The power switch current in the SC4503 is
internally limited to at least 1.4A. This is also the maximum
peak inductor or the peak input current. By estimating the
conduction losses in both the switch and the diode, an
expression of the maximum available output current of a
boost converter can be derived:
Example: Determine the highest attainable output voltage
when boosting from a single Li-ion cell.
ª
º
,
/,09
'
9' −'
(
9' − 9&(6$7
)
,1
,
=
ꢂ −
−
(4)
2870$;
«
»
9287
ꢀꢁ
9
,1
¬
¼
Equation (3) can be re-arranged as:
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
lessen jittery tendency but not so steep that large flux swing
decreases efficiency. For continuous-conduction mode
operation, inductor ripple current ΔIL between 0.35A and
0.6A is a good compromise. Setting ΔIL = 0.43A, VD = 0.5V
and f = 1.3MHz in (7),
9 −'9&(6$7
,1
9287
=
− 9'
(5)
ꢀ −'
Assuming that the voltage of a nearly discharged Li-ion cell
is 2.6V. Using VD=0.5V, VCESAT=0.3V and D=0.86 in (5),
§
·
§
·
9
9
9
9
,1
,1
,1
,1
¨
¨
¸
¸
¨
¨
¸
¸
/ =
ꢃ −
=
ꢃ −
(8)
I∆,/
9287 + 9'
ꢂꢁꢀꢄ
9287 + ꢂꢁꢀ
ꢇꢃꢁ − ꢄꢃꢅꢁ •ꢄꢃꢆ
©
¹
©
¹
9287
<
−ꢄꢃꢂ =ꢀꢁ9
ꢀ −ꢄꢃꢅꢁ
where L is in μH.
Equation (7) shows that for a given VOUT, ΔIL is the highest
Transient headroom requirement further reduces the maxi-
mum achievable output voltage to below 16V.
9287 + 9'
when
. If VIN varies over a wide range, then
9 =
Minimum Controllable On-Time
,1
ꢀ
choose L based on the nominal input voltage.
The operating duty cycle of a boost converter decreases as
VIN approaches VOUT. Sensed switch current ramp modulates
the pulse width in a current-mode switching regulator. This
current ramp is absent unless the switch is turned on. The
intersection of this ramp with the error amplifier output
determines the switch on-time. The propagation delay
time required to immediately turn off the switch after it is
turned on is the minimum controllable on time. Measured
minimum on time of the SC4503 is load-dependent and
ranges from 180ns to 220ns at room temperature. The
switch in the SC4503 is either not turned on, or, for at least
this minimum. If the regulator requires a switch on-time
less than this controllable minimum, then it will either skip
cycles or start to jitter.
The saturation current of the inductor should be 20-30%
higher than the peak current limit (1.9 A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Discontinuous-Conduction Mode
9287
0 =
The output-to-input voltage conversion ratio
in
9
,1
continuous-conduction mode is limited by the maximum
duty cycle DMAX
:
ꢀ
ꢀ
0 <
=
= ꢂꢁꢀ
ꢀ −'0$; ꢀ −ꢅꢁꢃꢄ
Inductor Selection
The inductor ripple current ΔIL ofa boost converter in con-
tinuous-conduction mode is
Higher voltage conversion ratios can be achieved by oper-
ating the boost converter in full-time discontinuous-con-
VOUT
'
9 − 9&(6$7
,1
duction mode (DCM). Define R =
as the equivalent
∆,/ =
(6)
IOUT
I/
output load resistance. The following inequalities must be
where f is the switching frequency and L is the induc-
tance.
satisfied for DCM operation:
Substituting (3) into (6) and neglecting VCESAT
,
/I 0−ꢂ
(9)
<
5
ꢁ0ꢀ
§
·
9
9
,1
,1
¨
¨
¸
¸
∆,/ =
ꢀ −
(7)
I/
9287 + 9'
and,
©
¹
9287 ꢂꢁꢀ$
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough to
,
=
<
(10)
287
5
0
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Switch on duty ratio in DCM is given by,
When the switch is turned on, the output capacitor supplies
the load current IOUT (Figure 4). The output ripple voltage
due to charging and discharging of the output capacitor
is therefore:
ꢃ/I
' =
0ꢂ0−ꢁꢀ
(11)
5
Higher input current ripples and lower output current are
the drawbacks of DCM operation.
,
287'7
∆9287
=
(13)
&
287
Input Capacitor
For most applications, a 10-22µF ceramic capacitor is suf-
The input current in a boost converter is the inductor cur- ficient for output filtering. It is worth noting that the output
rent, which is continuous with low RMS current ripples. A ripple voltage due to discharging of a 10µF ceramic capaci-
2.2-4.7µF ceramic input capacitor is adequate for most tor (13) is higher than that due to its ESR.
applications.
Rectifying Diode
Output Capacitor
For high efficiency, Schottky barrier diodes should be used
Both ceramic and low ESR tantalum capacitors can be as rectifying diodes for the SC4503. These diodes should
used as output filtering capacitors. Multi-layer ceramic have an average forward current rating at least equal to the
capacitors, due to their extremely low ESR (<5mΩ), are output current and a reverse blocking voltage of at least
the best choice. Use ceramic capacitors with stable a few volts higher than the output voltage. For switching
temperature and voltage characteristics. One may be regulators operating at low duty cycles (i.e. low output
tempted to use Z5U and Y5V ceramic capacitors for output voltage to input voltage conversion ratios), it is beneficial
filtering because of their high capacitance density and to use rectifying diodes with somewhat higher average cur-
small sizes. However these types of capacitors have high rent ratings (thus lower forward voltages). This is because
temperature and high voltage coefficients. For example, the diode conduction interval is much longer than that of
the capacitance of a Z5U capacitor can drop below 60% the transistor. Converter efficiency will be improved if the
of its room temperature value at –25°C and 90°C. X5R voltage drop across the diode is lower.
ceramic capacitors, which have stable temperature and
voltage coefficients, are the preferred type.
The rectifying diodes should be placed close to the SW
pin of the SC4503 to minimize ringing due to trace induc-
The diode current waveform in Figure 4 is discontinuous tance. Surface-mount equivalents of 1N5817 and 1N5818,
with high ripple-content. Unlike a buck converter in which MBRM120, MBR0520L, ZHCS400, 10BQ015 and equiva-
lent are suitable.
the inductor ripple current ∆IL determines the output ripple
voltage. The output ripple voltage of a boost regulator is
much higher and is determined by the absolute inductor
current. Decreasing the inductor ripple current does not
reduce the output ripple voltage appreciably. The current
flowing in the output filter capacitor is the difference
between the diode current and the output current. This
capacitor current has a RMS value of:
Shutdown and Soft-Start
The shutdown (
) pin is a dual function pin. When
SHDN SS
SHDN SS
driven from a logic gate with VOH>2V, the
functions as an on/off input to the SC4503. When the
shutdown pin is below 2V, it clamps the error amplifier
pin
96+'1 66
output to
and reduces the switch current limit.
SHDN SS
Connecting RSS and CSS to the
pin (Figure 5) slows
9287
,
−ꢀ
(12)
287
the voltage rise at the pin during start-up. This forces the
peak inductor current (hence the input current) to follow a
slow ramp, thus achieving soft-start.
9
,1
If a tantalum capacitor is used, then its ripple current rating
in addition to its ESR will need to be considered.
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
SHDN SS
The minimum
voltage for switching is 1.4V. The Pulling the SHDN SS pin below 0.4V shuts down the SC4503,
graph “Switch Current Limit vs. Shutdown Pin Voltage” in drawing less than 1µA from the input power supply. For
SHDN SS
voltages above 2V and below 0.4V, the
pin can be
SHDN SS
the “Typical Characteristics” shows that the
pin
regarded as a digital on/off input. Figure 5 shows several
ways of interfacing the control logic to the shutdown pin. In
Figure 5(a) soft-start is not used and the logic gate drives
the shutdown pin through a small ( ≈ 1kΩ ) optional resistor
RSS. RSS limits the current drawn by the SC4503 internal
voltage needs to be at least 2V for the SC4503 to deliver
its rated power. The effect of the SHDN SS voltage on the
SC4503 is analog between 1.4V and 2V. Within this range
the switch current limit is determined not by ILIM but in-
stead by the PWM signal path (see Figure 2). Moreover it
varies with duty cycle and the shutdown pin voltage.
V
IN
IN
IN
End of Soft-start
VSHDN/SS > 2V
SC4503
SC4503
VOH > 2V
RSS
V
OL < 0.4V
RLIM
SHDN/SS
SHDN/SS
CSS
(a)
(b)
V
IN
IN
End of Soft-start
VSHDN/SS > 2V
IN
1.7V < VOH < 2V
OL ≈ 0
RSS
SC4503
SC4503
V
OL < 0.4V
V
RSS
DSS
SHDN/SS
SHDN/SS
ISHDN/SS
ISHDN/SS
CMDSH-3
CSS
CSS
(c)
(d)
V
V
IN
IN
IN
IN
End of Soft-start
VSHDN/SS > 2V
SC4503
SC4502
1N4148
RSS
VOH > VIN
SHDN/SS
SHDN/SS
RSS
CSS
CSS
(e)
(f)
Figure 5. Methods of Driving the Shutdown Pin and Soft-starting the SC4503
(a) Directly Driven from a Logic Gate. RLIM Limits the Gate Output Current during Fault,
(b) Soft-start Only,
(c) Driven from a Logic Gate with Soft-start,
(d) Driven from a Logic Gate with Soft-start (1.7V < VOH < 2V),
(e) Driven from an Open-collector NPN Transistor with Soft-start and
(f) Driven from a Logic Gate (whose VOH > VIN) with Soft-start.
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
ꢀ,287
ꢀ
Output filter pole, ωSꢀ = −
= −
,
circuit from the driving logic gate during fault condition.
In Figure 5(f) the shutdown pin is driven from a logic gate
whose VOH is higher than the supply voltage to the SC4503.
The diode clamps the maximum shutdown pin voltage to
one diode voltage above the input power supply.
9287
&
5&ꢀ
ꢀ
ꢀ
Compensating zero, ω=ꢀ = −
and
5&&&
ꢁ
5 ꢀ −'
( )
ω
=
Right half plane (RHP) zero,
.
=ꢁ
During soft-start, CSS is charged by the difference between
/
,
the RSS current and the shutdown pin current, 6+'1 66. In
steady state, the voltage drop across RSS reduces the shut-
down pin voltage according to the following equation:
I
OUT
V
IN
POWER
STAGE
V
OUT
96+'1 66 = 9(1 −566,6+'1 66
(14)
ESR
C2
R
C4
R1
R2
FB
In order for the SC4503 to achieve its rated switch current,
96+'1 66
-
COMP
Gm
must be greater than 2V in steady state. This
puts an upper limit on RSS for a given enable voltage VEN (=
+
RC
CC
1.252V
RO
,
voltage applied to RSS). The maximum specified
is
6+'1 66
VOLTAGE
REFERENCE
9
= ꢀ9
(see “Electrical Characteristics”).
50µA with
6+'1 66
The largest RSS can be found using (14):
RO is the equivalent output resistance of the error amplifier
9(1ꢄ0,1ꢃ −ꢂ
566
<
Figure 6. Simplified Equivalent Model of a Boost
ꢀꢁµ$
Converter
If the enable signal is less than 2V, then the interfacing
options shown in Figures 5(d) and 5(e) will be preferred. The
methods shown in Figures 5(a) and 5(c) can still be used
The poles p1, p2 and the RHP zero z2 all increase phase
shift in the loop response. For stable operation, the over-
all loop gain should cross 0dB with -20dB/decade slope.
Due to the presence of the RHP zero, the 0dB crossover
ω]ꢁ
however the switch current limit will be reduced. Variations
,
6+'1 66 and switch current limit with
SHDN SS
pin voltage
of
frequency should not be more than
. The internal
ꢀ
and temperature are shown in the “Typical Characteristics”.
Shutdown pin current decreases as temperature increases.
compensating zero z1 provides phase boost beyond p2. In
general the converter is more stable with widely spaced
filter pole p2 and the RHP zero z2. The RHP zero moves to
low frequency when either the duty-cycle D or the output
current IOUT increases. It is beneficial to use small inductors
and larger output capacitors especially when operating at
9287
9
Switch current limit at a given
also decreases as
6+'1 66
temperature rises. Lower shutdown pin current flowing
through RSS at high temperature results in higher shutdown
pin voltage. However reduction in switch current limit (at
9
a given 6+'1 66 ) at high temperature is the dominant
high
ratios.
effect.
9
,1
A feed-forward capacitor C4 is needed for stability. The value
of C4 can be determined empirically by observing the induc-
tor current and the output voltage during load transient.
Feed-Forward Compensation
Figure 6 shows the equivalent circuit of a boost converter.
Important poles and zeros of the overall loop response
are:
ꢀꢂꢁµV
ꢃꢂꢁµV
Starting with a value between
and
, C4 is
5ꢀ
5ꢀ
adjusted until there is no excessive ringing or overshoot in
inductor current and output voltage during load transient.
Sizing the inductor such that its ripple current is about 0.5A
also improves phase margin and transient response.
ꢀ
ωSꢀ = −
Low frequency integrator pole,
,
52&&
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Figures 7(a)-7(c) show the effects of different values of Board Layout Considerations
inductance and feed-forward capacitance on transient re-
sponses. In a battery-operated system if C4 is optimized for In a step-up switching regulator, the output filter capacitor,
the minimum VIN and the maximum load step, the converter the main power switch and the rectifying diode carry pulse
will be stable over the entire input voltage range.
currents with high di/dt. For jitter-free operation, the size of
the loop formed by these components should be minimized.
Since the power switch is integrated inside the SC4503,
grounding the output filter capacitor next to the SC4503
ground pin minimizes size of the high di/dt current loop.
The input bypass capacitors should also be placed close to
the input pins. Shortening the trace at the SW node reduces
the parasitic trace inductance. This not only reduces EMI
but also decreases switching voltage spikes.
VOUT
0.5V/div
IL1
0.5A/div
Figure 8 shows how various external components are
placed around the SC4503.
40µs/div
(a) L1 = 5.6µH and C4 = 2.2pF
The large surrounding ground plane acts as a heat sink
for the device.
VOUT
0.5V/div
VOUT
VIN
D1
L1
IL1
0.5A/div
SW
JP
C2
C1
R1
C4
U1
40µs/div
R3
FB
R2
(b) L1 = 5.6µH and C4 = 3.3pF
C3
SHDN/SS
GND
VOUT
0.5V/div
Figure 8. Suggested PCB Layout for the SC4503.
IL1
0.5A/div
40µs/div
(c) L1 = 3.3µH and C4 = 2.7pF
Figure 7. Different inductances and feed-forward capaci-
tancesaffecttheloadtransientresponsesofthe
3.3V to 12V step-up converter in Figure 10(a).
IOUT is switched between 90mA and 280mA.
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Typical Application Circuits
L1
D1
5V
10µH
ZHCS400
+
R3
54.9k
D2
24V
_
MM5Z24VT1
5
1
C2
0.22µF
C1
4.7µF
C4
IN
SW
SC4503
220pF
R4
4
3
SHDN/SS
GND
FB
301k
C5
22nF
C3
56nF
2
R1
63.4
R2
63.4
L1: Murata LQH32C
C1: Murata GRM219R60J475K
Figure 9. Driving Two 6 White LED Strings from 5V. Zener diode D2 protects the converter
from over-voltage damage when both LED strings become open.
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SC4503
POWER MANAGEMENT
Typical Application Circuits
D1
L1
VIN
VOUT
12V
3.3V
2.7µH
10BQ015
5
1
C4
2.2pF
R3
15k
R1
866k
IN
SW
SC4503
C1
2.2µF
C2
4.7µF
3
4
SHDN/SS
GND
FB
C3
56nF
R2
100k
2
L1: Coiltronics LD1
C1: Murata GRM188R61A225K
C2: Murata GRM21BR61C475K
Figure 10(a). 3.3V to 12V Boost Converter with Soft-start
Efficiency vs Load Current
95
90
85
80
75
70
65
60
55
50
1.3MHz
VOUT = 12V
40µs/div
0.001
0.010
0.100
1.000
Upper Trace : Output Voltage, AC Coupled, 0.5V/div
Lower Trace : Input Inductor Current, 0.5A/div
Load Current (A)
Figure 10(b). Efficiency vs Load Current
Figure 10(c). Load Transient Response of the Circuit
in Figure 10(a). IOUT is switched between
90mA and 280mA
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SC4503
POWER MANAGEMENT
Typical Application Circuits
Efficiency vs Load Current
D1
L1
VOUT
5V
2.6 - 4.2V
95
90
85
80
75
70
65
60
55
50
1.5µH
3.3V
OFF ON
10BQ015
5
1
C4
10pF
< 0.4V
R1
187k
VIN = 4.2V
IN
SW
R3
15k
1-CELL
LI-ION
SC4503
SHDN/SS
C1
4.7µF
C2
10µF
3
4
VIN = 3.6V
VIN = 2.6V
FB
GND
2
C3
56nF
R2
60.4k
VOUT = 5V
1.3MHz
L1: TDK VLF4012AT
0.001
0.010
0.100
1.000
C1: Murata GRM188R60J475K
C2: Murata GRM21BR60J106K
Load Current (A)
Figure 11(a). Single Li-ion Cell to 5V Boost Converter
Figure 11(b). Efficiency of the Li-ion Cell to 5V
Boost Converter
VIN = 2.6V
VIN = 4.2V
40µs/div
40µs/div
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Inductor Current, 0.5A/div
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Inductor Current, 0.5A/div
Figure 11(c). Load Transient Response. IOUT is switched
between 0.1A and 0.5A
Figure 11(d). Load Transient Response. IOUT is switched
between 0.15A and 0.9A
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SC4503
POWER MANAGEMENT
Typical Application Circuits
VOUT
3.3V, 0.45A
C5
L1
D1
2.6 - 4.2V
3.3µH
2.2µF
10BQ015
5
1
C4
15pF
R1
412k
R3
8.06k
L2
3.3µH
IN
SW
SC4503
1-CELL
LI-ION
C1
1µF
C2
10µF
4
3
SHDN/SS
GND
FB
R2
249k
C3
2
0.22µF
L1 and L2: Coiltronics DRQ73-3R3
C1: Murata GRM188R61A105K
C2: Murata GRM21BR60J106K
C5: Murata GRM188R61A225K
Figure 12(a). Single Li-ion Cell to 3.3V SEPIC Converter.
Efficiency vs Load Current
85
80
75
70
65
60
55
50
45
40
35
30
VOUT = 3.3V
VIN = 3.6V
VIN = 2.6V
VIN = 3.6V
VIN = 4.2V
40µs/div
0.001
0.010
0.100
1.000
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Input Inductor Current, 0.2A/div
Load Current (A)
Figure 12(b). Efficiency vs Load Current
Figure 12(c). Load Transient Response of the Circuit
in Figure 12(a). IOUT is switched between
100mA and 500mA
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SC4503
POWER MANAGEMENT
Typical Application Circuits
D2
D3
D4
D5
OUT2
26V (10mA)
C5
0.1µF
C6
0.1µF
C7
0.1µF
C8
1µF
D1
L1
3.3V
OUT1
4.7µH
9V (0.3A)
10BQ015
R1
309k
5
1
C4
IN
SW
12pF
3.3V
OFF ON
< 0.4V
SC4503
C2
4.7µF X 2
4
3
R3
SHDN/SS
GND
FB
RUN
17.8k
C1
4.7µF
R2
49.9k
C9
0.1µF
C3
2
56nF
D7
OUT3
-8.5V (10mA)
C10
1µF
D6
D2 - D7 : BAT54S
L1 : Sumida CDC5D23B-4R7M
C2: Murata GRM21BR61C475K
C1: Murata GRM188R61A105K
Figure 13(a). Triple-Output TFT Power Supply with Soft-Start
CH4
CH1
CH2
CH3
400µs/div
40µs/div
CH1 : OUT1 Voltage, 5V/div
CH2 : OUT2 Voltage, 20V/div
CH3 : OUT3 Voltage, 5V/div
CH4 : RUN Voltage, 5V/div
Upper Trace : Output Voltage, AC Coupled, 0.5V/div
Lower Trace : Inductor Current, 0.5A/div
Figure 13(b). TFT Power Supply Start-up Transient as
the RUN Voltage is Stepped from 0 to
3.3V
Figure 13(c).
Load Transient Response. IOUT1
is switched between 50mA and
350mA
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
EVB Schematic
D1
SS13
L1
4.7uH
12VOUT
5VIN
U1
C1
10uF
R1
0R
R2
432K
C2
N.P.
C3
10uF
8
7
6
1
2
3
N.C.
SW
SW
VIN
GND
GND
C4
15pF
R3
47K
OFF/ON
5
4
FB
SHDN/SS
R4
0R
R5
49.9K
C5
100nF
JP1
SC4503_MLPD
L1
4.7uH
D1
SS13
12VOUT
5VIN
1
2
3
5
SW
VIN
R1
0R
C2
N.P.
C3
10uF
R2
432K
R3
47K
C1
10uF
GND
C4
15pF
OFF/ON
4
FB SHDN
U1
SC4503
R4
0R
R5
49.9K
C5
100n
JP1
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Outline Drawing - TSOT-23
DIMENSIONS
INCHES MILLIMETERS
A
DIM
A
MIN NOM MAX MIN NOM MAX
-
-
-
-
-
-
-
-
-
-
-
-
.039
1.00
0.10
0.90
0.50
0.20
e1
D
E
A1 .000
A2 .028
.004 0.00
.035 0.70
.020 0.30
.008 0.08
b
c
D
.012
.003
N
2X E/2
ccc
E1
.110 .114 .118 2.80 2.90 3.00
E1 .060 .063 .067 1.50 1.60 1.70
1
2
E
e
e1
L
.110 BSC
.037 BSC
.075 BSC
2.80 BSC
0.95 BSC
1.90 BSC
C
2X N/2 TIPS
.012 .018 .024 0.30 0.45 0.60
e
(.024)
5
-
(0.60)
5
-
L1
N
01
aaa
bbb
ccc
B
0°
8°
0°
8°
.004
.008
.010
0.10
0.20
0.25
D
aaa
C
A2
A
SEATING
PLANE
H
A1
C
c
bxN
bbb
GAGE
PLANE
C
A-B D
0.25
L
01
(L1)
DETAIL A
SEE DETAIL A
SIDE VIEW
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H-
3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS
OR GATE BURRS.
4. REFERENCE JEDEC STD MO-193, VARIATION AB.
Land Pattern - TSOT-23
DIMENSIONS
X
DIM
INCHES
(.087)
.031
MILLIMETERS
(2.20)
0.80
0.95
0.60
1.40
3.60
C
G
P
X
Y
Z
(C)
G
Z
.037
.024
.055
.141
Y
P
NOTES:
1.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Outline Drawing - 8 Lead 2X2mm MLPD-W
B
E
A
D
DIMENSIONS
INCHES MILLIMETERS
MIN NOM MAX MIN NOM MAX
.028 .031 0.70 0.75 0.80
DIM
A
.030
PIN 1
INDICATOR
(LASER MARK)
A1 .000 .001 .002 0.00 0.02 0.05
(.008)
(0.20)
A2
b
D
.007 .010 .012 0.18 0.25 0.30
.075 .079 .083 1.90 2.00 2.10
D1 .059 .063 .067 1.50 1.60 1.70
E
.075 .079 .083 1.90 2.00 2.10
E1 .031 .035 .039 0.80 0.90 1.00
A
C
e
.020 BSC
0.50 BSC
0.30
8
SEATING
PLANE
L
.008 .012 .016 0.20
0.40
aaa
C
N
8
aaa
.003
.003
0.08
0.08
A1
A2
bbb
D1
1
2
LxN
E/2
E1
N
bxN
bbb
C A B
e
e/2
D/2
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
Land Pattern - 8 Lead 2X2mm MLPD-W
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
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