MAX1556A [MAXIM]
16μA IQ, 1.2A PWM Step-Down DC-DC Converters; 16レ一个智商， 1.2A PWM降压型DC- DC转换器
| 型号： | MAX1556A |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 16μA IQ, 1.2A PWM Step-Down DC-DC Converters |
| 文件： | 总12页 (文件大小：276K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3336; Rev 2; 6/10
16µA I , 1.2A PWM
Q
Step-Down DC-DC Converters
6/MAX157
General Description
Features
The MAX1556/MAX1556A/MAX1557 are low-operating-
current (16µA), fixed-frequency step-down regulators.
High efficiency, low-quiescent operating current, low
dropout, and minimal (27µA) quiescent current in
dropout make these converters ideal for powering
portable devices from 1-cell Li-ion or 3-cell alkaline/NiMH
batteries. The MAX1556 delivers up to 1.2A; has pin-
selectable 1.8V, 2.5V, and 3.3V outputs; and is also
adjustable. The MAX1557 delivers up to 600mA; has pin-
selectable 1V, 1.3V, and 1.5V outputs; and is also
adjustable.
o Up to 97% Efficiency
o 95% Efficiency at 1mA Load Current
o Low 16µA Quiescent Current
o 1MHz PWM Switching
o Tiny 3.3µH Inductor
o Selectable 3.3V, 2.5V, 1.8V, 1.5V, 1.3V, 1.2V, 1.0V,
and Adjustable Output
o 1.2A Guaranteed Output Current
(MAX1556/MAX1556A)
The MAX1556/MAX1556A/MAX1557 contain a low-on-
resistance internal MOSFET switch and synchronous
rectifier to maximize efficiency and dropout perfor-
mance while minimizing external component count. A
proprietary topology offers the benefits of a high fixed-
frequency operation while still providing excellent effi-
ciency at both light and full loads. A 1MHz PWM
switching frequency keeps components small. Both
devices also feature an adjustable soft-start to minimize
battery transient loading.
o Voltage Positioning Optimizes Load-Transient
Response
o Low 27µA Quiescent Current in Dropout
o Low 0.1µA Shutdown Current
o No External Schottky Diode Required
o Analog Soft-Start with Zero Overshoot Current
o Small, 10-Pin, 3mm x 3mm TDFN Package
The MAX1556/MAX1556A/MAX1557 are available in a
tiny 10-pin TDFN (3mm x 3mm) package.
Ordering Information
TOP
MARK
Applications
PART
TEMP RANGE PIN-PACKAGE
PDAs and Palmtop Computers
Cell Phones and Smart Phones
Digital Cameras and Camcorders
Portable MP3 and DVD Players
Hand-Held Instruments
MAX1556ETB+
-40°C to +85°C 10 TDFN-EP*
ACQ
AUJ
ACR
MAX1556AETB+ -40°C to +85°C 10 TDFN-EP*
MAX1557ETB+ -40°C to +85°C 10 TDFN-EP*
*EP = Exposed paddle.
+Denotes a lead(Pb)-free/RoHS-compliant package.
Typical Operating Circuit
Pin Configuration
OUTPUT
0.75V TO V
TOP VIEW
INPUT
2.6V TO 5.5V
IN
INP
LX
MAX1556
MAX1556A
MAX1557
1
2
3
4
5
IN
10 D1
GND
SS
INP
LX
9
8
7
6
IN
PGND
MAX1556
MAX1556A
MAX1557
D1
D2
VOLTAGE
SELECT
PGND
D2
OUT
SHDN
OUT
SS
ON
OFF
SHDN
GND
TDFN
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
ABSOLUTE MAXIMUM RATINGS
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
Soldering Temperature (reflow) .......................................+260°C
IN, INP, OUT, D2, SHDN to GND ..........................-0.3V to +6.0V
SS, D1 to GND.............................................-0.3V to (V + 0.3V)
IN
PGND to GND .......................................................-0.3V to +0.3V
LX Current (Note 1)........................................................... 2.25A
Output Short-Circuit Duration.....................................Continuous
Continuous Power Dissipation (T = +70°C)
A
10-Pin TDFN (derate 24.4mW/°C above +70°C) .......1951mW
Note 1: LX has internal clamp diodes to GND and IN. Applications that forward bias these diodes should take care not to exceed
the IC’s package power-dissipation limits.
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
= 3.6V, T = - 40°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.) (Note 1)
SHDN A A
(V = V
IN
= V
INP
PARAMETER
Input Voltage
CONDITIONS
MIN
2.6
TYP
MAX
5.5
2.55
25
UNITS
V
V
Undervoltage-Lockout Threshold
V
rising and falling, 35mV hysteresis (typ)
2.20
2.35
16
IN
No switching, D1 = D2 = GND
Dropout
Quiescent Supply Current
µA
27
42
T
= +25°C
= +85°C
0.1
0.1
1
A
A
Shutdown Supply Current
Output Voltage Range
SHDN = GND
µA
V
T
0.75
-0.25
-0.75
-1.5
V
IN
No load
+0.75
0
+1.75
+0.75
0
300mA load
T
A
= 0°C to +85°C
600mA load
-0.75
-2.25
-2.25
(Note 2)
6/MAX157
1200mA load, MAX1556
1200mA load, MAX1556A
No load
-2.75
-1.25
Output Accuracy
%
-0.75
-1.5
+2.25
+1.5
+0.50
-1.0
300mA load
T
A
= -40°C to +85°C
(Note 2)
600mA load
-2.25
-4.0
1200mA load, MAX1556
MAX1556/MAX1556A
MAX1557
1200
600
Maximum Output Current
OUT Bias Current
mA
µA
T
T
= +25°C
= +85°C
0.01
0.01
3
0.1
A
D1 = D2 = GND
MAX1556/MAX1557
A
For preset output voltages
D1 = D2 = GND, No load
= 0.75V at
4.5
-0.50
-1.2
+0.75
0
+1.75
+1.2
V
OUT
300mA load
300mA (typ),
= 0°C to +85°C
600mA load
-1.75
-3.25
-1.25
-1.75
-2.75
-4.25
-0.75
-2.25
+0.25
-1.25
+2.25
+1.50
+0.25
-1.00
T
A
MAX1556/MAX1557
1200mA load, MAX1556 only
No load
FB Threshold Accuracy
%
D1 = D2 = GND,
V
= 0.75V
OUT
300mA load
at 300mA (typ),
= -40°C to +85°C
600mA load
T
A
1200mA load, MAX1556 only
MAX1556/MAX1557
2
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
6/MAX157
ELECTRICAL CHARACTERISTICS (continued)
= 3.6V, T = - 40°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.) (Note 1)
SHDN A A
(V = V
IN
= V
INP
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
= 2.6V to 3.6V
-0.37
MAX1556,
IN
D1 = IN, D2 = GND;
MAX1556A
D1 = D2 = IN
V
= 3.6V to 5.5V
0.33
IN
Line Regulation
%
V
V
V
V
V
V
= 2.6V to 3.6V
= 3.6V to 5.5V
= 3.6V
-0.1
0.09
0.19
0.23
0.35
0.42
0.27
0.33
1.8
IN
IN
IN
IN
IN
IN
MAX1557,
D1 = IN, D2 = GND
0.35
0.7
MAX1556/MAX1556A
MAX1557
= 2.6V
p-Channel On-Resistance
n-Channel On-Resistance
Ω
= 3.6V
= 2.6V
V
V
= 3.6V
= 2.6V
0.48
IN
IN
Ω
A
MAX1556/MAX1556A
MAX1557
1.5
0.8
2.1
1.2
p-Channel Current-Limit
Threshold
1.0
n-Channel Zero Crossing
Threshold
20
35
45
mA
MAX1556/MAX1556A
MAX1557
1.8
1.0
10
RMS LX Output Current
LX Leakage Current
A
RMS
T
= +25°C
= +85°C
0.1
0.1
A
V
= 5.5V, LX =
IN
µA
GND or IN
T
A
Maximum Duty Cycle
100
%
%
Minimum Duty Cycle
0
Internal Oscillator Frequency
SS Output Impedance
0.9
1
200
90
1.1
300
200
MHz
kΩ
Ω
∆V / I for I = 2µA
130
SS SS
SS
SS Discharge Resistance
Thermal-Shutdown Threshold
SHDN = GND, 1mA sink current
+160
15
°C
Thermal-Shutdown Hysteresis
LOGIC INPUTS (D1, D2, SHDN)
Input-Voltage High
°C
2.6V ≤ V ≤ 5.5V
1.4
V
V
IN
Input-Voltage Low
0.4
1
T
T
= +25°C
= +85°C
0.1
0.1
A
Input Leakage
µA
A
Note 1: All units are 100% production tested at T = +25°C. Limits over the operating range are guaranteed by design.
A
Note 2: For the MAX1556, 3.3V output accuracy is specified with a 4.2V input.
_______________________________________________________________________________________
3
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
Typical Operating Characteristics
(V = V
IN
= 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, T = +25°C, unless otherwise noted.)
INP
A
EFFICIENCY vs. LOAD CURRENT
WITH 3.3V OUTPUT
EFFICIENCY vs. LOAD CURRENT
WITH 2.5V OUTPUT
EFFICIENCY vs. LOAD CURRENT
WITH 1.8V OUTPUT
100
90
80
70
60
50
100
90
80
70
60
50
40
100
90
80
70
60
50
40
V
= 5V
IN
V
= 4.2V
IN
V = 5V
IN
V
= 3.6V
V
= 3.6V
IN
IN
V
= 5V
IN
V
= 3V
IN
V
= 2.6V
IN
V
= 3.6V
IN
V
= 2.6V
V
= 3V
IN
IN
40
0.1
1
10
100
1000
10,000
0.1
1
10
100
1000 10,000
0.1
1
10
100
1000 10,000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT
WITH 1.0V OUTPUT (MAX1557)
OUTPUT VOLTAGE
vs. LOAD CURRENT
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH 600mA LOAD
1.84
1.83
1.82
1.81
1.80
1.79
1.78
1.77
1.76
1.75
1.74
1.789
1.788
1.787
1.786
1.785
1.784
1.783
1.782
1.781
1.780
1.779
100
90
80
70
60
50
T
= -45°C
A
V
= 5V
IN
V
= 3.6V
IN
T
= -40°C
A
T
= +25°C
T
= +25°C
V
= 3V
A
A
IN
6/MAX157
V
= 2.6V
IN
T
= +85°C
A
T
= +85°C
A
40
0
200
400
600
800 1000 1200
2.5
3.0
3.5
4.0
4.5
5.0
5.5
0.1
1
10
100
1000
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH NO LOAD
SUPPLY CURRENT vs. INPUT VOLTAGE
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1556/7 toc09
1.812
1.811
1.810
1.809
1.808
1.807
1.806
20
18
16
14
12
10
8
I
= 750mA
LOAD
V
OUT
AC-COUPLED
10mV/div
T
= -40°C
A
T
= +25°C
A
V
LX
2V/div
0
6
T
= +85°C
A
I
LX
1.805
1.804
1.803
4
500mA/div
0
2
0
400ns
2.5
3.0
3.5
4.0
4.5
5.0
5.5
1
2
3
4
5
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
4
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
6/MAX157
Typical Operating Characteristics (continued)
(V = V
IN
= 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, T = +25°C, unless otherwise noted.)
A
INP
EXTERNAL FEEDBACK
SWITCHING WAVEFORMS
LIGHT-LOAD SWITCHING WAVEFORMS
SOFT-START/SHUTDOWN WAVEFORMS
MAX1556/7 toc10b
MAX1556/7 toc10
MAX1556/7 toc11
5V/div
0
V
SHDN
20mV/div
V
OUT
AC-COUPLED
V
1V/div
0
OUT
V
2V/div
0
C
R
= 470pF
LX
LX
SS
= 4Ω
LOAD
V
2V/div
LX
LX
500mA/div
0
I
LX
0
200mA/div
0
200mA/div
0
I
500mA/div
0
I
I
IN
V
= 5V, V
= 3.3V, I
= 500mA
IN
OUT
OUT
2µs/div
4µs/div
100µs/div
LOAD TRANSIENT
SOFT-START RAMP TIME vs. C
SS
MAX1556/7 toc13
10
50mV/div
AC-COUPLED
V
OUT
1
500mA/div
0
I
OUT
I
= 20mA
OUTMIN
0.1
20µs/div
0
500
1000
1500
(pF)
2000
2500
C
SS
LOAD TRANSIENT
LINE TRANSIENT
BODE PLOT
MAX1556/7 toc14
MAX1556/7 toc15
MAX1556/7 toc16
40
30
240
210
180
150
120
90
4V
V
20
IN
50mV/div
AC-COUPLED
3.5V
V
OUT
10
0
-10
-20
-30
-40
10mV/div
AC-COUPLED
V
OUT
0dB
PHASE MARGIN = 53°
60
30
500mA/div
0
200mA/div
0
0
I
I
OUT
LX
-50
C
-60
-30
-60
I
= 180mA
= 22µF, R
= 4Ω
OUTMIN
OUT
LOAD
20µs/div
40µs/div
0.1
1
10
100
1000
FREQUENCY (kHz)
_______________________________________________________________________________________
5
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
Pin Description
PIN
1
NAME
IN
FUNCTION
Supply Voltage Input. Connect to a 2.6V to 5.5V source.
Ground. Connect to PGND.
2
GND
Soft-Start Control. Connect a 1000pF capacitor (C ) from SS to GND to eliminate input-current
SS
overshoot during startup. C is required for normal operation of the MAX1556/MAX1557. For greater
SS
3
SS
than 22µF total output capacitance, increase C to C
SS
/ 22,000 for soft-start. SS is internally
OUT
discharged through 200Ω to GND in shutdown.
Output Sense Input. Connect to the output of the regulator. D1 and D2 select the desired output
voltage through an internal feedback resistor-divider. The internal feedback resistor-divider remains
connected in shutdown.
4
5
OUT
Shutdown Input. Drive SHDN low to enable low-power shutdown mode. Drive high or connect to IN
for normal operation.
SHDN
6
7
D2
OUT Voltage-Select Input. See Table 1.
Power Ground. Connect to GND.
PGND
Inductor Connection. Connected to the drains of the internal power MOSFETs. High impedance in
shutdown mode.
8
LX
Supply Voltage, High-Current Input. Connect to a 2.6V to 5.5V source. Bypass with a 10µF ceramic
capacitor to PGND.
9
INP
D1
EP
10
—
OUT Voltage-Select Input. See Table 1.
Exposed Paddle. Connect to ground plane. EP also functions as a heatsink. Solder to circuit-board
ground plane to maximize thermal dissipation.
Control Scheme
Table 1. Output-Voltage-Select Truth Table
During PWM operation the converters use a fixed-fre-
quency, current-mode control scheme. The heart of the
current-mode PWM controller is an open-loop, multiple-
input comparator that compares the error-amp voltage
feedback signal against the sum of the amplified cur-
rent-sense signal and the slope-compensation ramp. At
the beginning of each clock cycle, the internal high-side
p-channel MOSFET turns on until the PWM comparator
trips. During this time the current in the inductor ramps
up, sourcing current to the output and storing energy in
the inductor’s magnetic field. When the p-channel turns
off, the internal low-side n-channel MOSFET turns on.
Now the inductor releases the stored energy while the
current ramps down, still providing current to the output.
The output capacitor stores charge when the inductor
current exceeds the load and discharges when the
inductor current is lower than the load. Under overload
conditions, when the inductor current exceeds the cur-
rent limit, the high-side MOSFET is turned off and the
low-side MOSFET remains on until the next clock cycle.
6/MAX157
MAX1556
MAX1556A
MAX1557
D1
D2
V
OUT
V
OUT
V
OUT
Adjustable
Adjustable
(V = 0.75)
FB
from 0.75V to
(V = 0.75)
FB
from 0.75V to
0
0
3.3V
V
V
IN
IN
0
1
1
1
0
1
3.3V
2.5V
1.8V
1.5V
1.2V
2.5V
1.5V
1.3V
1.0V
A zero represents D_ being driven low or connected to GND.
A 1 represents D_ being driven high or connected to IN.
Detailed Description
The MAX1556/MAX1557 synchronous step-down con-
verters deliver a guaranteed 1.2A/600mA at output volt-
ages from 0.75V to V . They use a 1MHz PWM
IN
current-mode control scheme with internal compensation,
allowing for tiny external components and a fast transient
response. At light loads the MAX1556/MAX1557 automat-
ically switch to pulse-skipping mode to keep the quies-
cent supply current as low as 16µA. Figures 2 and 3
show the typical application circuits.
6
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
6/MAX157
SHORT-CIRCUIT
PROTECTION
CLOCK
1MHz
IN
BIAS
SHDN
CURRENT-LIMIT
COMPARATOR
INP
V
CS
CURRENT
SENSE
PWM
AUTO SKIP
CONTROL
0.675V
PWM
COMPARATOR
LX
SLOPE
COMP
PGND
SKIP-OVER
ENTER SKIP/
SR OFF
ZERO-CROSS
DETECT
ERROR
AMPLIFIER
OUT
GND
REFERENCE
1.25V
D1
D2
OUTPUT
VOLTAGE
SELECTOR
MAX1556
MAX1556A
MAX1557
SS
Figure 1. Functional Diagram
OUTPUT
OUTPUT
L1
3.3µH
L2
4.7µH
0.75V TO V
0.75V TO V
INPUT
IN
INPUT
2.6V TO 5.5V
IN
2.6V TO 5.5V
1.2A
600mA
INP
IN
LX
INP
LX
C1
10µF
C4
10µF
R1
100Ω
C5
22µF
C2
22µF
MAX1556
MAX1556A
MAX1557
PGND
IN
PGND
C4
0.47µF
D1
D2
D1
D2
VOLTAGE
SELECT
VOLTAGE
SELECT
OUT
SS
OUT
SS
ON
ON
C6
1000pF
C3
1000pF
OFF
SHDN
SHDN
OFF
GND
GND
Figure 2. MAX1556 Typical Application Circuit
Figure 3. MAX1557 Typical Application Circuit
_______________________________________________________________________________________
7
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
As the load current decreases, the converters enter a
pulse-skip mode in which the PWM comparator is dis-
abled. At light loads, efficency is enhanced by a
pulse-skip mode in which switching occurs only as
needed to service the load. Quiescent current in skip
mode is typically 16µA. See the Light-Load Switching
Waveforms and Load Transient graphs in the Typical
Operating Characteristics.
1.0
0.5
0
V
= 3.6V
IN
V
= 5.5V
-0.5
-1.0
-1.5
-2.0
-2.5
IN
V
= 2.6V
IN
Load-Transient Response/
Voltage Positioning
The MAX1556/MAX1556A/MAX1557 match the load
regulation to the voltage droop seen during transients.
This is sometimes called voltage positioning. The load
line used to achieve this behavior is shown in Figures 4
and 5. There is minimal overshoot when the load is
removed and minimal voltage drop during a transition
400
600
800 1000
0
200
1200
LOAD CURRENT (mA)
Figure 4. MAX1556 Voltage-Positioning Load Line
from light load to full load. Additionally, the MAX1556,
MAX1556A, and MAX1557 use a wide-bandwidth feed-
back loop to respond more quickly to a load transient
than regulators using conventional integrating feedback
loops (see Load Transient in the Typical Operating
Characteristics).
1.0
0.8
0.6
0.4
The MAX1556/MAX1556A/MAX1557 use of a wide-band
control loop and voltage positioning allows superior
load-transient response by minimizing the amplitude
and duration of overshoot and undershoot in response
to load transients. Other DC-DC converters, with high
gain- control loops, use external compensation to main-
tain tight DC load regulation but still allow large voltage
droops of 5% or greater for several hundreds of
microseconds during transients. For example, if the
load is a CPU running at 600MHz, then a dip lasting
100µs corresponds to 60,000 CPU clock cycles.
V
= 3.6V
IN
0.2
0
V
= 5.5V
IN
-0.2
-0.4
-0.6
-0.8
-1.0
V
IN
= 2.6V
6/MAX157
0
200
400
600
LOAD CURRENT (mA)
Voltage positioning on the MAX1556/MAX1556A/
MAX1557 allows up to 2.25% (typ) of load-regulation
voltage shift but has no further transient droop. Thus,
during load transients, the voltage delivered to the CPU
remains within spec more effectively than with other
regulators that might have tighter initial DC accuracy. In
summary, a 2.25% load regulation with no transient
droop is much better than a converter with 0.5% load
regulation and 5% or more of voltage droop during load
transients. Load-transient variation can be seen only
with an oscilloscope (see the Typical Operating
Characteristics), while DC load regulation read by a
voltmeter does not show how the power supply reacts
to load transients.
Figure 5. MAX1557 Voltage-Positioning Load Line
MOSFET is always on. This is particularly useful in
battery-powered applications with a 3.3V output. The sys-
tem and load might operate normally down to 3V or less.
The MAX1556/MAX1556A/MAX1557 allow the output to
follow the input battery voltage as it drops below the regu-
lation voltage. The quiescent current in this state rises
minimally to only 27µA (typ), which aids in extending bat-
tery life. This dropout/100% duty-cycle operation achieves
long battery life by taking full advantage of the entire bat-
tery range.
The input voltage required to maintain regulation is a
function of the output voltage and the load. The differ-
ence between this minimum input voltage and the out-
put voltage is called the dropout voltage. The dropout
voltage is therefore a function of the on-resistance of
Dropout/100% Duty-Cycle Operation
The MAX1556/MAX1556A/MAX1557 function with a low
input-to-output voltage difference by operating at 100%
duty cycle. In this state, the high-side p-channel
the internal p-channel MOSFET (R
inductor resistance (DCR).
) and the
DS(ON)P
8
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
6/MAX157
Table 2. Inductor Selection
MANUFACTURER
Taiyo Yuden
Taiyo Yuden
TOKO
PART
LMNP04SB3R3N
LMNP04SB4R7N
D52LC
VALUE (µH)
DCR (mΩ)
36
I
(mA)
SIZE (mm)
5 x 5 x 2.0
SHIELDED
Yes
SAT
3.3
4.7
3.5
4.7
4.7
4.7
4.7
4.7
4.7
1300
50
1200
1340
1140
1200
1200*
790
5 x 5 x 2.0
Yes
73
5 x 5 x 2.0
Yes
TOKO
D52LC
87
5 x 5 x 2.0
Yes
Sumida
CDRH3D16
D412F
50
3.8 x 3.8 x 1.8
4.8 x 4.8 x 1.2
2.5 x 3.2 x 2.0
3.0 x 3.2 x 1.7
2.8 x 3.2 x 1.5
Yes
TOKO
100*
97
Yes
Murata
LQH32CN
CXL180
No
Sumitomo
Sumitomo
70*
1000*
800*
No
CXLD140
100*
No
*Estimated based upon similar-valued prototype inductors.
V
= I
x (R
+ DCR)
Thermal Shutdown
DROPOUT
OUT
DS(ON)P
As soon as the junction temperature of the
MAX1556/MAX1556A/MAX1557 exceeds +160°C, the
ICs go into thermal shutdown. In this mode the internal
p-channel switch and the internal n-channel synchro-
nous rectifier are turned off. The device resumes nor-
mal operation when the junction temperature falls
below +145°C.
R
is given in the Electrical Characteristics. DCR
DS(ON)P
for a few recommended inductors is listed in Table 2.
Soft-Start
The MAX1556/MAX1556A/MAX1557 use soft-start to
eliminate inrush current during startup, reducing tran-
sients at the input source. Soft-start is particularly use-
ful for higher-impedance input sources such as Li+ and
alkaline cells. Connect the required soft-start capacitor
from SS to GND. For most applications using a 22µF
output capacitor, connect a 1000pF capacitor from SS
to GND. If a larger output capacitor is used, then use
the following formula to find the value of the soft-start
capacitor:
Applications Information
The MAX1556/MAX1556A/MAX1557 are optimized for
use with small external components. The correct selec-
tion of inductors and input and output capacitors
ensures high efficiency, low output ripple, and fast tran-
sient response.
C
22000
OUT
Adjusting the Output Voltage
The MAX1556/MAX1556A/MAX1557 offer preset output
voltages of 1.0V, 1.2V, 1.3V, 1.5V, 1.8V, 2.5V, and 3.3V
as well as an adjustable output using external resistors.
Whenever possible, the preset outputs (set by D1 and
D2) should be used. With external resistor feedback,
noise coupling to FB can cause alternate LX pulse to
terminate early resulting in an inductor current wave-
form with alternate large and small current pulses. See
the External Feedback Switching Waveforms graph in
Typical Operating Characteristics section). Note that
external feedback and the alternating large-small pulse
waveform do not impact loop stability and have no
harmful effect on regulation or reliability.
C
=
SS
Soft-start is implemented by exponentially ramping up
the output voltage from 0 to V with a time con-
OUT(NOM)
stant equal to C
times 200kΩ (see the Typical
SS
Operating Characteristics). Assuming three time con-
stants to full output voltage, use the following formula to
calculate the soft-start time:
3
t
= 600 x 10 x C
SS
SS
Shutdown Mode
Connecting SHDN to GND or logic low places the
MAX1556/MAX1556A/MAX1557 in shutdown mode and
reduces supply current to 0.1µA. In shutdown, the con-
trol circuitry and the internal p-channel and n-channel
MOSFETs turn off and LX becomes high impedance.
Connect SHDN to IN or logic high for normal operation.
The adjustable output is selected when D1 = D2 = 0
and an external resistor-divider is used to set the output
voltage (see Figure 6). The MAX1556/MAX1557 have a
defined line- and load-regulation slope. The load regu-
lation is for both preset and adjustable outputs and is
described in the Electrical Characteristics table and
Figures 4 and 5. The impact of the line-regulation slope
_______________________________________________________________________________________
9
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
can be reduced by applying a correction factor to the
feedback resistor equation.
OUTPUT
R2
First, calculate the correction factor, k, by plugging the
desired output voltage into the following formula:
ERROR
AMPLIFIER
OUT
⎛
⎞
V
− 0.75V
3.6V
−2
OUTPUT
k = 1.06 x 10 V x
⎜
⎟
R3
REFERENCE
1.25V
⎝
⎠
k represents the shift in the operating point at the feed-
back node (OUT).
Select the lower feedback resistor, R3, to be ≤35.7kΩ to
ensure stability and solve for R2:
⎛
⎞
0.75V −k
R3
R3+R2
=
⎜
⎟
V
⎝
⎠
OUTPUT
SS
Inductor Selection
Figure 6. Adjustable Output Voltage
A 4.7µH inductor with a saturation current of at least
800mA is recommended for the MAX1557 full-load
(600mA) application. For the MAX1556/MAX1556A appli-
cation with 1.2A full load, use a 3.3µH inductor with at
least 1.34A saturation current. For lower full-load cur-
rents the inductor current rating can be reduced. For
maximum efficiency, the inductor’s resistance (DCR)
should be as low as possible. Please note that the core
material differs among different manufacturers and
inductor types and has an impact on the efficiency. See
Table 2 for recommended inductors and manufacturers.
most applications. The input capacitor can be increased
for better input filtering.
IN Input Filter
In all MAX1557 applications, connect INP directly to IN
and bypass INP as described in the Input Capacitor
section. No additional bypass capacitor is required at
IN. For applications using the MAX1556 and
MAX1556A, an RC filter between INP and IN keeps
power-supply noise from entering the IC. Connect a
100Ω resistor between INP and IN, and connect a
0.47µF capacitor from IN to GND.
6/MAX157
Capacitor Selection
Ceramic input and output capacitors are recommend-
ed for most applications. For best stability over a wide
temperature range, use capacitors with an X5R or bet-
ter dielectric due to their small size, low ESR, and low
temperature coefficients.
Soft-Start Capacitor
The soft-start capacitor, C , is required for proper
SS
operation of the MAX1556/MAX1556A/MAX1557. The
recommended value of C
is discussed in the Soft-
SS
Start section. Soft-start times for various soft-start
capacitors are shown in the Typical Operating
Characteristics.
Output Capacitor
The output capacitor C
is required to keep the out-
OUT
put voltage ripple small and to ensure regulation loop
stability. C must have low impedance at the switch-
ing frequency. A 22µF ceramic output capacitor is rec-
ommended for most applications. If a larger output
capacitor is used, then paralleling smaller capacitors is
suggested to keep the effective impedance of the
capacitor low at the switching frequency.
OUT
PCB Layout and Routing
Due to fast-switching waveforms and high-current
paths, careful PCB layout is required. An evaluation kit
(MAX1556EVKIT) is available to speed design.
When laying out a board, minimize trace lengths
between the IC, the inductor, the input capacitor, and
the output capacitor. Keep these traces short, direct,
and wide. Keep noisy traces, such as the LX node
trace, away from OUT. The input bypass capacitors
should be placed as close as possible to the IC.
Connect GND to the exposed paddle and star PGND
and GND together at the output capacitor. The ground
connections of the input and output capacitors should
be as close together as possible.
Input Capacitor
Due to the pulsating nature of the input current in a buck
converter, a low-ESR input capacitor at INP is required
for input voltage filtering and to minimize interference
with other circuits. The impedance of the input capacitor
should be kept very low at the switching frequen-
cy. A minimum value of 10µF is recommended at INP for
C
INP
10 ______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
6/MAX157
Chip Information
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PROCESS: BiCMOS
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
10 TDFN
T1033-1
21-0137
90-0003
______________________________________________________________________________________ 11
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
CHANGED
DESCRIPTION
0
1
7/04
3/08
Initial release
Adding MAX1556A as a new version
—
1–12
Added soldering temperature, added TOC for external feedback switching
waveforms, and added paragraph discussing noise coupling when using
external feedback resistors
1, 2, 5, 6, 9,
10, 11
2
6/10
6/MAX157
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
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