MAX1708 [MAXIM]
High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter; 高频率，大功率，低噪声，升压型DC- DC转换器
| 型号： | MAX1708 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | High-Frequency, High-Power, Low-Noise, Step-Up DC-DC Converter |
| 文件： | 总13页 (文件大小：333K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2068; Rev 0; 7/01
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
General Description
Features
The MAX1708 sets a new standard of space savings for
high-power, step-up DC-DC conversion. It delivers up
to 10W at a fixed (3.3V or 5V) or adjustable (2.5V to
5.5V) output, using an on-chip power MOSFET from a
+0.7V to +5V supply.
o On-Chip 5A Power MOSFET
o 5V, 2A Output from a 3.3V Input
o Fixed 3.3V or 5V Output Voltage or
Adjustable (2.5V to 5.5V)
Fixed-frequency PWM operation ensures that the
switching noise spectrum is constrained to the 600kHz
fundamental and its harmonics, allowing easy postfilter-
ing for noise reduction. External clock synchronization
capability allows for even tighter noise spectrum con-
trol. Quiescent power consumption is less than 1mW to
extend operating time in battery-powered systems.
o Input Voltage Range Down to 0.7V
o Low Power Consumption
1mW Quiescent Power
1µA Current in Shutdown Mode
o Low-Noise, Constant Frequency Operation
(600kHz)
Two control inputs (ONA, ONB) allow simple push-on,
push-off control through a single momentary push-but-
ton switch, as well as conventional on/off logic control.
The MAX1708 also features programmable soft-start
and current limit for design flexibility and optimum per-
formance with batteries. The maximum RMS switch cur-
rent rating is 5A. For a device with a higher (10A)
switch current rating, refer to the MAX1709 data sheet.
o Synchronizable Switching Frequency
(350kHz to 1000kHz)
o Small QSOP Package
________________________Applications
Ordering Information
Routers, Servers, Workstations, Card Racks
Local 2.5V to 3.3V or 5V Conversion
Local 3.3V to 5V Conversion
PART
TEMP. RANGE
PIN-PACKAGE
MAX1708EEE
-40°C to +85°C
16 QSOP
3.6V or 5V RF PAs in Communications Handsets
Typical Operating Circuit
Pin Configuration
INPUT
1V TO 5V
TOP VIEW
ONB
ONA
LX
1
2
3
4
5
6
7
8
16 CLK
2.2µH
15 3.3/5
14 PGND
OUTPUT
3.3V, 5V,
OR ADJ
OFF ON
ONA
CLK
LX
LX
MAX1708
13 PGND
12 PGND
11 FB
UP TO 2A
SYNC
OR
INTERNAL
MAX1708
LX
GND
OUT
GND
SS/LIM
REF
SS/LIM
REF
10 OUT
9
GND
QSOP
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
ONA, ONB, OUT, SS/LIM, 3.3/5 to GND ...............-0.3V to +6.0V
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
LX to PGND ...........................................................-0.3V to +6.0V
FB, CLK, REF to GND.............................. -0.3V to (V
+ 0.3V)
OUT
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (T = +70°C)
A
16-Pin QSOP (derate 8.30mW/°C above +70°C). .......667mW
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
= V
= +3.6V, ONA = ONB = FB = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
OUT
CLK
A
A
PARAMETER
CONDITIONS
3.3/5 = GND, I
MIN
3.26
4.90
TYP
3.34
5.05
-0.40
1.240
1
MAX
3.42
5.20
-0.60
1.265
200
UNITS
= 0.5A
= 0.5A
SW
SW
Output Voltage
V
< 0.1V (Note 1)
V
FB
3.3/5 = OUT, I
Load Regulation
Measured between 0.5A < I
< 1.5A (Note 2)
%/A
V
SW
FB Regulation Voltage (V
FB Input Current
)
I
= 0.5A
1.215
FB
SW
V
= +1.5V
nA
V
FB
Output Voltage Adjust Range
2.5
2.0
40
5.5
Output Undervoltage Lockout
(Note 3)
Rising and falling
=1.5V
2.3
V
Frequency in Startup Mode
Minimum Startup Voltage
Minimum Operating Voltage
Soft-Start Pin Current
V
400
1.1
kHz
V
OUT
I
< 1mA, T = +25°C (Note 4)
0.9
0.7
4
OUT
A
(Note 5)
V
V
V
= 1V
3.2
5.0
µA
µA
SS/LIM
OUT Supply Current
= 1.5V (Note 6)
200
300
FB
OUT Leakage Current In
Shutdown
V
V
= 3.6V
0.1
1
2
µA
µA
ONB
LX Leakage Current
= V
= V = +5.5V
OUT
25
80
LX
ONB
N-Channel Switch
On-Resistance
30
mΩ
4.5
7.0
3.85
5
SS/LIM = open
SS/LIM = 150kΩ to GND
5.3
N-Channel Current Limit
A
1.80
3.00
RMS Switch Current
A
RMS
Reference Voltage
I
= 0
1.245
1.260
4
1.275
10
V
REF
Reference Load Regulation
Reference Supply Rejection
-1µA ≤ I
≤ 50µA
mV
mV
REF
+2.5V ≤ V
≤ +5.5V
0.2
5
OUT
✕
ONA, ONB, 3.3/5, 1.2V < V
< 5.5V
0.2
0.2
V
V
OUT
OUT
OUT
Input Low Level (Note 7)
V
✕
CLK, 2.7V < V
< 5.5V
OUT
2
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V
= V
= +3.6V, ONA = ONB = FB = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
OUT
CLK
A
A
PARAMETER
CONDITIONS
ONA, ONB, 3.3/5, 1.2V < V <5.5V
MIN
TYP
MAX
UNITS
✕
0.8
V
OUT
OUT
OUT
Input High Level
V
✕
CLK, 2.7 V< V
< 5.5V
0.8
V
OUT
Logic Input Current
ONA, ONB, CLK, 3.3/5 = 0, 5.5V
-1
1
µA
kHz
%
Internal Oscillator Frequency
Maximum Duty Cycle
520
82
600
88
680
94
External Clock Frequency
Range
350
100
1000
kHz
CLK Pulse Width
(Note 8)
(Note 8)
ns
ns
CLK Rise/Fall Time
50
ELECTRICAL CHARACTERISTICS
(V
= V = +3.6V, ONA = ONB = FB = GND, T = -40°C to +85°C, unless otherwise noted.) (Note 9)
CLK A
OUT
PARAMETER
CONDITIONS
3.3/5 = GND, I
< 0.1V, V = +2.4V
IN
MIN
3.24
4.90
1.20
MAX
3.45
5.22
1.28
200
UNITS
= 0.5A
SW
V
FB
Output Voltage
V
(Note 1)
3.3/5 = OUT, I = 0.5A
SW
FB Regulation Voltage
I
= 0.5A
V
SW
FB Input Current (V
Load Regulation
)
FB
V
= +1.5V
FB
nA
Measured between 0.5A < I
< 1.5A (Note 2)
-0.60
5.2
%/A
µA
SW
Soft-Start Pin Current
SS/LIM = 1V
3.2
OUT Leakage Current in
Shutdown
V
V
= 3.6V
2
µA
µA
ONB
OUT Supply Current
= 1.5V (Note 6)
300
80
FB
N-Channel Switch
On-Resistance
mΩ
SS/LIM = open
4.5
1.8
7.5
4.0
N-Channel Current Limit
Reference Voltage
A
V
SS/LIM = 150kΩ to GND
I
= 0
1.24
1.28
REF
_______________________________________________________________________________________
3
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V
= V = +3.6V, ONA = ONB = FB = GND, T = -40°C to +85°C, unless otherwise noted.) (Note 9)
CLK A
OUT
PARAMETER
CONDITIONS
MIN
MAX
UNITS
✕
ONA, ONB, 3.3/5, 1.2V < V
< 5.5V
0.2
0.2
V
V
OUT
OUT
OUT
Input Low Level (Note 7)
Input High Level
V
✕
CLK, 2.7V < V
< 5.5V
OUT
✕
ONA, ONB, 3.3/5, 1.2V < V
CLK, 2.7V < V < 5.5V
< 5.5V
0.8
V
OUT
OUT
OUT
V
✕
0.8
V
OUT
Logic Input Current
ONA, ONB, CLK, 3.3/5 = 0, 5.5V
-1
1
µA
kHz
%
Internal Oscillator Frequency
Maximum Duty Cycle
External Clock Frequency Range
CLK Pulse Width
500
80
700
95
350
100
1000
kHz
ns
(Note 8)
(Note 8)
CLK Rise/Fall Time
50
ns
✕
Note 1: Output voltage is specified at 0.5A switch current I , which is equivalent to approximately 0.5A (V / V ) of load cur-
OUT
SW
IN
rent.
Note 2: Load regulation is measured by forcing specified switch current and straight-line calculation of change in output voltage in
✕
external feedback mode. Note that the equivalent load current is approximately I
(V / V
).
OUT
SW
IN
Note 3: Until undervoltage lockout is reached, the device remains in startup mode. Do not apply full load until this voltage is
reached.
Note 4: Startup is tested with Figure 1’s circuit. Output current is measured when both the input and output voltages are applied.
Note 5: Minimum operating voltage. The MAX1708 is bootstrapped and will operate down to a 0.7V input once started.
Note 6: Supply current is measured from the output voltage (+3.3V) to the OUT pin. This correlates directly with actual input supply
current but is reduced in value according to the step-up ratio and efficiency.
Note 7: ONA and ONB inputs have approximately 0.15V hysteresis.
Note 8: Guaranteed by design, not production tested.
Note 9: Specifications to -40°C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Typical Operating Characteristics
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. SWITCHING FREQUENCY
100
80
60
40
20
0
100
80
60
40
20
0
90
89
88
87
86
V
= 2.5V
IN
V
= 3.3V
IN
V
IN
= 2.5V
V
= 1.2V
IN
V
= 5V
V
= 3.3V
1
OUT
OUT
V
= 3.3V, V
= 5V, I
=1A
IN
OUT
OUT
0.1
1
10
100
1000 10,000
0.1
10
100
1000 10,000
350 450 550 650 750 850 950
SWITCHING FREQUENCY (kHz)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
LINE REGULATION
LOAD REGULATION
(V = 2.5V, V
= 3.3V)
(V = 5V)
OUT
IN
OUT
2.0
1.5
1.0
0.3
0.2
0.1
0
2.0
1.5
1.0
0.5
0
0.5
0
-0.5
-1.0
-0.5
-1.0
-0.1
-0.2
-0.3
I
= 1A
OUT
I
= 500mA
OUT
-1.5
-2.0
-1.5
-2.0
PLOT NORMALIZED TO V = 3.3V
PLOT NORMALIZED TO I
= 500mA
PLOT NORMALIZED TO I
= 500mA
IN
OUT
OUT
0.1
1
10
100
1000 10,000
2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00
INPUT VOLTAGE (V)
0.1
1
10
100
1000 10,000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
LINE REGULATION
(V
= 3.3V)
OUT
70
60
50
40
30
20
10
0
0.8
25
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
I
= 1A
OUT
V
= 5V, V DECREASING
IN
20
15
10
5
OUT
V
OUT
= 5V, V INCREASING
IN
V
OUT
= 3.3V, V INCREASING
IN
V
OUT
= 3.3V, V DECREASING
IN
-0.1
-0.2
-0.3
I
= 500mA
OUT
PLOT NORMALIZED TO V = 2.5V
IN
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6
1.50 1.75 2.00 2.25 2.50 2.75 3.00
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
STARTUP VOLTAGE vs. LOAD CURRENT
(V = 3.3V)
STARTUP VOLTAGE vs. LOAD CURRENT
(V = 5V)
SWITCHING FREQUENCY
vs. TEMPERATURE
OUT
OUT
2.5
2.0
1.5
1.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
2
1
PLOT NORMALIZED TO 25°C
T
= -40°C
A
T
T
= +25°C
= +85°C
A
T
= -40°C
= +25°C
A
0
A
T
A
T
A
= +85°C
-1
-2
0.5
0
V
OUT
= 3.3V
1
10
100
1000
10,000
-40
-15
10
35
60
85
1
10
100
1000
10,000
LOAD CURRENT (mA)
TEMPERATURE (°C)
LOAD CURRENT (mA)
SWITCH CURRENT LIMIT
vs. TEMPERATURE
SWITCH CURRENT LIMIT
vs. SS/LIM RESISTANCE
NOISE vs. FREQUENCY
4000
5
4
3
2
1
0
6.0
5.5
5.0
4.5
4.0
3500
3000
2500
2000
1500
1000
500
RESOLUTION = 1kHz
V
IN
= 3.3V, V
-15
= 5V
OUT
0
0.1
1
10
-40
10
35
60
85
0
50
100
150
200
250
300
FREQUENCY (MHz)
TEMPERATURE (°C)
SS/LIM RESISTANCE (kΩ)
LINE TRANSIENT RESPONSE
HEAVY SWITCHING WAVEFORM
MAX1708 toc12
MAX1708 toc11
3.5V
5V
0
V
V
IN
LX
5V/div
500mV/div
3V
4A
2A
I
L
2A/div
V
5V
OUT
AC-COUPLED
50mV/div
0
V
OUT
5V
AC-COUPLED
50mV/div
100µs/div
1µs/div
6
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
STARTUP WITHOUT SOFT-START
LOAD TRANSIENT RESPONSE
(C = 0)
SS
MAX1708 toc14
MAX1708 toc13
5V
0
4A
V
I
ONA
5V/div
SW
2A/div 2A
0
2A
1A
V
5V
0
I
OUT
AC-COUPLED
IN
1A/div
50mV/div
0
2A
1A
0
R = 5Ω
L
I
OUT
4V
2V
V
OUT
2V/div
1A/div
2ms/div
40µs/div
STARTUP WITH SOFT-START
STARTUP WITH SOFT-START
(C = 0.01µF)
(C = 0.1µF)
SS
SS
MAX1708 toc15
MAX1708 toc16
5V
0
5V
0
V
V
ONA
5V/div
ONA
5V/div
1A
2A
I
IN
1A/div
1A
0
I
IN
1A/div
0
4V
2V
0
4V
V
OUT
V
OUT
2V/div
2V/div
2V
0
R = 5Ω
L
R = 5Ω
L
2ms/div
2ms/div
_______________________________________________________________________________________
7
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Pin Description
PIN
1
NAME
ONB
FUNCTION
Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).
On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1).
Drain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect
2
ONA
3, 4, 5
6, 9
LX
an external Schottky diode from LX to V
. (Figure 1)
OUT
GND
Ground
Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the
rate at which the device reaches current limit (soft-start). To reduce the current limit from the
preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During
shutdown, SS/LIM is internally pulled to GND to discharge the soft-start capacitor.
7
SS/LIM
Voltage Reference Output. Bypass with a 0.22µF capacitor to GND. Maximum REF load is
50µA.
8
REF
Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1µF capacitor
to PGND with less than 5mm trace length. Connect a 2Ω series resistor from the output filter
capacitor (0.1µF) to OUT (Figure 1).
10
OUT
DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see
3.3/5 pin description). Connect a resistor-divider from the output to set the output voltage in the
+2.5V to +5.5V range. FB regulates to +1.24V (Figure 4).
11
12, 13, 14
15
FB
PGND
3.3/5
CLK
Power Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14
together with wide traces.
Output Voltage Selection Input. When FB is connected to GND, the regulator uses internal
feedback to set the output voltage. 3.3/5 = low sets output to 3.3V; 3.3/5 = high sets output to
5V. If an external divider is used at FB, connect 3.3/5 to ground.
Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK with
an external clock for external synchronization.
16
8
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
L1
2.2µH
D1
V
V
OUT
+5V
IN
KEEP TRACES
SHORT AND WIDE
LX
LX
LX
C2
150µF
C1
150µF
R2
2Ω
ONA
ONB
ON/OFF
CONTROL
GND
GND
CLK
R1
C3
3.3/5
MAX1708
OUT
SS/LIM
C5
0.1µF
C4
0.22µF
FB
REF
GND
GND
KEEP TRACES
SHORT AND WIDE
PGND
PGND PGND
Figure 1. Standard Operating Circuit
Implement inverted single-line on/off control by ground-
ing ONA and toggling ONB. Implement momentary
pushbutton on/off as described in the Applications
Information section. Both inputs have approximately
0.15V of hysteresis.
_______________Detailed Description
The MAX1708 step-up converter offers high efficiency
and high integration for high-power applications. It
operates with an input voltage as low as 0.7V and is
suitable for single- to 3-cell battery inputs, as well as
2.5V or 3.3V regulated supply inputs. The output volt-
age is preset to +3.3V or +5.0V or can be adjusted with
external resistors for voltages between +2.5V to +5.5V.
Switching Frequency
The MAX1708 switches at the fixed-frequency internal
oscillator rate (600kHz) or can be synchronized to an
external clock. Connect CLK to OUT for internal clock
operation. Apply a clock signal to CLK to synchronize
to an external clock. The MAX1708 will synchronize to a
new external clock rate in two cycles and will take
approximately 40µs to revert to its internal clock fre-
quency once the external clock pulses stop and CLK is
driven high. Table 2 summarizes oscillator operation.
The MAX1708 internal N-channel MOSFET switch is
rated for 5A (RMS value) and can deliver loads to 2A,
depending on input and output voltages. For flexibility,
the current limit and soft-start rate are independently
programmable.
A 600kHz switching frequency allows for a small induc-
tor to be used. The switching frequency is also syn-
chronizable to an external clock ranging from 350kHz
to 1MHz.
Operation
The MAX1708 switches at a constant frequency
(600kHz) and modulates the MOSFET switch pulse
width to control the power transferred per cycle and
regulate the voltage across the load. In low-noise appli-
cations, the fundamental and the harmonics generated
by the fixed switching frequency are easily filtered out.
Figure 2 shows the simplified functional diagram for the
MAX1708. Figure 3 shows the simplified PWM con-
ONA, ONB
The logic levels at ONA and ONB turn the MAX1708 on
or off. When ONA = 1 or ONB = 0, the device is on.
When ONA = 0 and ONB = 1, the device is off (Table
1). Logic high on-control can be implemented by con-
necting ONB high and using ONA for shutdown.
_______________________________________________________________________________________
9
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Table 1. On/Off Logic Control
Table 2. Selecting Switching Frequency
ONA
ONB
MAX1708
On
CLK
MODE
Not allowed
PWM
0
0
1
1
0
1
0
1
0
1
Off
External clock
(350kHz−1000kHz)
On
Synchronized PWM
On
UNDERVOLTAGE LOCKOUT
OUT
MAX1708
IC POWER
2.15V
PWM
CONTROLLER
STARTUP
OSCILLATOR
EN
EN
Q
D
ONA
EN
ON
REFERENCE
RDY
LX
ONB
REF
1.26V
OSC
600kHz
OSCILLATOR
N
CLK
PGND
FB
FB
3.3/5
GND
DUAL MODE
FB
OUT
Figure 2. Simplified Functional Diagram
VOUT
VFB
troller functional diagram. The MAX1708 enters syn-
chronized current-mode PWM when a clock signal
R3 = R4
−1
(350kHz < f
< 1MHz) is applied to CLK. For wire-
CLK
less or noise-sensitive applications, this ensures that
switching harmonics are predictable and kept outside
the IF frequency band(s). High-frequency operation
permits low-magnitude output ripple voltage and mini-
mum inductor and filter capacitor size. Switching loss-
es will increase at higher frequencies (see MAX1708 IC
Power Dissipation).
where V = 1.24V.
FB
Soft-Start/Current Limit Adjustment
(SS/LIM)
The soft-start pin allows the soft-start time to be adjust-
ed by connecting a capacitor from SS/LIM to GND.
Select capacitor C3 (see Figure 1):
✕
t
= 4ms + [110 C3 (in µF)]
SS
Setting the Output Voltage
The MAX1708 features Dual Mode™ operation. When
FB is connected to ground, the MAX1708 generates a
fixed output voltage of either +3.3V or +5V, depending
on the logic applied to the 3.3/5 input (Figure 1). The
output can be configured for other voltages, using two
external resistors as shown in Figure 4. To set the out-
put voltage externally, choose an R3 value that is large
enough to minimize load at the output but small enough
to minimize errors due to leakage and the time constant
to FB. A value of R4 ≤ 50kΩ is required
where t is the time (in milliseconds) it takes output to
SS
reach its final value.
To improve efficiency or reduce inductor size at
reduced load currents, the current limit can be reduced
from its nominal value (see Electrical Characteristics).
A resistor (R1 in Figure 1) between SS/LIM and ground
reduces the current limit as follows:
I
1
R1 = 312kΩ ×
ILIM
where I is the desired current limit in amperes and R1
1
Dual Mode is a trademark of Maxim Integrated Products.
≤ 312kΩ. I
= 5A, if R1 is omitted.
LIM
10 ______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Table 3. Component Selection Guide
PRODUCTION
INDUCTORS
Coiltronics UP2B-2R2
Coilcraft DO3316P-222HC
CAPACITORS
Sanyo 6TPC100M
Panasonic EEFUE0J151R
DIODES
Motorola MBRD1035CTL
Central CMSH5-20
Surface mount
internal current limit. Note that this current may be
reduced through SS/LIM if less than the MAX1708’s full
load current is needed (see Electrical Characteristics
for ratings). For highest efficiency, use a coil with low
DC resistance, preferably under 20mΩ. To minimize
radiated noise, use a toroid, pot core, or shielded
inductor. See Tables 3 and 4 for a list of recommended
components and component suppliers. To calculate
the maximum output current (in amperes), use the fol-
lowing equation:
Table 4. Component Suppliers
SUPPLIER
Central
PHONE
FAX
631-435-1110
847-639-6400
561-241-7876
602-303-5454
714-373-7939
631-435-1824
847-639-1489
561-241-9339
602-994-6430
714-373-7183
Coilcraft
Coiltronics
Motorola
Panasonic
__________________Design Procedure
V
+ V − V
2× ƒ ×L1
OUT D IN
I
= D' I
− D'
OUT(MAX)
LIM
Inductor Selection (L1)
The MAX1708’s high switching frequency allows the
use of a small-size inductor. Use a 2.2µH inductor for
600kHz operation. If the MAX1708 is synchronized at a
different frequency, scale the inductor value with the
✕
where:
V
V
= input voltage
IN
D
= forward voltage drop of the Schottky diode
at I
LIM
inverse of frequency (L = 2.2µH 600kHz / f
).
SYNC
1
V
= output voltage
OUT
D' = (V ) / (V
The PWM design tolerates inductor values within 25%
of this calculated value, so choose the closest standard
inductor value. For example, use 3.3µH for 350kHz and
1.5µH for 1MHz).
+ V ), neglecting switch voltage
D
IN
drop
OUT
f = switching frequency
L1 = inductor value
Inductors with a ferrite core or equivalent are recom-
mended; powder iron cores are not recommended for
use at high switching frequencies. Ensure the induc-
tor’s saturation rating (the current at which the core
begins to saturate and inductance falls) exceeds the
I
= minimum value of switch current limit from
Electrical Characteristics or set by R1 of
Figure 1.
LIM
V
IN
FB
REF
SLOPE
COMP
LX
Q
R
S
N
V
OUT
LX
FB
MAX1708
R4
SS/LIM
12.5
11mΩ
(LIMITED TO 100mV)
R3
PGND
KEEP SHORT
OSCILLATOR
Figure 3. Simplified PWM Controller Functional Diagram
Figure 4. Adjustable Output Voltage
______________________________________________________________________________________ 11
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
MAX1708 IC Power Dissipation
The major components of MAX1708 dissipated power
are switch conductance loss (P ), capacitive loss
SW
µC
270kΩ
(P
), and switch transition loss (P
). Numerical
TRAN
CAP
MAX1708
examples provided in brackets ({ }) correspond to the
following condition:
ON/OFF
ONB
ONA
V
DD
I/O
I/O
{V = 3.3V, V
IN
= 5V, V = 0.5V, I
= 2A}
OUT
OUT
D
An important parameter to compute the power dissipat-
ed in the MAX1708 is the approximate peak switch cur-
rent (I ):
SW
270kΩ
I
0.1µF
OUT
D'
I
=
{3.33A}
SW
V
IN
+ V
D' =
{0.6}
V
Figure 5. Momentary Pushbutton On-Off Switch
OUT
D
P
= P
+ P
+ P {0.472W}
TRAN
Diode Selection (D1)
The MAX1708’s high switching frequency demands a
high-speed rectifier. Use Schottky diodes (Table 3).
The diode’s current rating must exceed the maximum
load current, and its breakdown voltage must exceed
D
SW
CAP
2
✕
P
= (1 - D') I
R {0.353W}
SW
SW
SW
P
= (C
+ C
+ C
) (V
+ V )2f {0.045W}
CAP
DIO
DSW
GSW
OUT
D
✕
✕
P
= (V
+ V ) I
t
SW
f / 3 {0.073W}
TRAN
OUT
D
SW
V
. The diode must be placed within 10mm of the
OUT
where:
LX switching node and the output filter capacitor. The
diode also must be able to dissipate the power calcu-
lated by the following equation:
R
SW
= switch resistance {80mΩ}
= catch-diode capacitance {500pF}
= switch drain capacitance {1250pF}
C
C
C
DIO
✕
P
= I
V
D
DSW
GSW
DIODE
OUT
= switch gate capacitance {750pF}
where I
is the average load current and V is the
D
OUT
diode forward voltage at the peak switch current.
f = switching frequency {600kHz}
= switch turn-on or turn-off time {20ns}
t
SW
Capacitor Selection
Input Bypass Capacitor (C1)
A 150µF, low-ESR input capacitor will reduce peak cur-
rents and reflected noise due to inductor current ripple.
Lower ESR allows for lower input ripple current, but
combined ESR values up to 100mΩ are acceptable.
Smaller ceramic capacitors may also be used for light
loads or in applications that can tolerate higher input
current ripple.
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn
the MAX1708 on and off. As shown in Figure 5, when
ONA is pulled low and ONB is pulled high, the device
is off. When the momentary switch is pressed, ONB is
pulled low and the regulator turns on. The switch
should be on long enough for the microcontroller to exit
reset. The controller issues a logic high to ONA, which
guarantees that the device will stay on regardless of
the subsequent switch state. To turn the regulator off,
depress the switch long enough for the controller to
read the switch status and pull ONA low. When the
switch is released, ONB pulls high and the regulator
turns off.
Output Filter Capacitor (C2)
The output filter capacitor ESR must be kept under
30mΩ for stable operation. Polymer capacitors of
150µF (Panasonic EEFUE0J151R) typically exhibit
10mΩ of ESR. This translates to approximately 35mV of
output ripple at 3.5A switch current. Bypass the
MAX1708 IC supply input (OUT) with a 0.1µF ceramic
capacitor to GND and a 2Ω series resistor (R2, as
shown in Figure 1).
Layout Considerations
Due to high inductor current levels and fast switching
waveforms, proper PC board layout is essential. Protect
12 ______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
sensitive analog grounds by using a star ground config-
accomplished with a large PGND plane on the surface of
the board. Also note that outer-layer ground plane area
beneath the device provides little heat-sinking benefit. If
an outer-layer ground plane is not feasible, the PGND
pins should be connected to the inner-layer ground
plane with multiple vias (at least three vias per pin is rec-
ommended). Since the purpose of these vias is to opti-
mize thermal conductivity to the inner ground plane, be
sure that the vias have no gaps in their connections to
the ground plane. Refer to a layout example in the
MAX1708EVKIT data sheet.
uration. Connect PGND, the input bypass capacitor
ground lead, and the output filter capacitor ground lead
to a single point (star ground configuration). In addition,
minimize trace lengths to reduce stray capacitance and
trace resistance, especially from the LX pins to the catch
diode (D1) and output capacitor (C2) to PGND pins. If an
external resistor-divider is used to set the output voltage
(Figure 4), the trace from FB to the resistors must be
extremely short and must be shielded from switching
signals, such as CLK or LX. To optimize package power
dissipation and minimize device heating under heavy
loads, expand PC trace area connected to the three
PGND pins as much as the layout can allow. This is best
___________________Chip Information
TRANSISTOR COUNT: 1112
SUBSTRATE: GND
PROCESS: BiCMOS
Package Information
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2001 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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