Maxim MAX1556 16î¼a iq, 1.2a pwm step-down dc-dc converter Datasheet

19-3336; Rev 2; 6/10
KIT
ATION
EVALU
E
L
B
A
AVAIL
16µA IQ, 1.2A PWM
Step-Down DC-DC Converters
The MAX1556/MAX1556A/MAX1557 are low-operatingcurrent (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 pinselectable 1.8V, 2.5V, and 3.3V outputs; and is also
adjustable. The MAX1557 delivers up to 600mA; has pinselectable 1V, 1.3V, and 1.5V outputs; and is also
adjustable.
The MAX1556/MAX1556A/MAX1557 contain a low-onresistance internal MOSFET switch and synchronous
rectifier to maximize efficiency and dropout performance while minimizing external component count. A
proprietary topology offers the benefits of a high fixedfrequency operation while still providing excellent efficiency 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.
The MAX1556/MAX1556A/MAX1557 are available in a
tiny 10-pin TDFN (3mm x 3mm) package.
Applications
PDAs and Palmtop Computers
Features
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)
o Voltage Positioning Optimizes Load-Transient
Response
o
o
o
o
Low 27µA Quiescent Current in Dropout
Low 0.1µA Shutdown Current
No External Schottky Diode Required
Analog Soft-Start with Zero Overshoot Current
o Small, 10-Pin, 3mm x 3mm TDFN Package
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
TOP
MARK
MAX1556ETB+
-40°C to +85°C 10 TDFN-EP*
Cell Phones and Smart Phones
MAX1556AETB+
-40°C to +85°C 10 TDFN-EP*
AUJ
Digital Cameras and Camcorders
MAX1557ETB+
-40°C to +85°C 10 TDFN-EP*
ACR
Portable MP3 and DVD Players
*EP = Exposed paddle.
+Denotes a lead(Pb)-free/RoHS-compliant package.
Hand-Held Instruments
Pin Configuration
Typical Operating Circuit
INPUT
2.6V TO 5.5V
OUTPUT
0.75V TO VIN
INP
IN
VOLTAGE
SELECT
TOP VIEW
LX
MAX1556
MAX1556A
MAX1557
PGND
IN
1
GND
2
SS
3
OUT
4
SHDN
5
D1
OUT
D2
ON
OFF
ACQ
SS
MAX1556
MAX1556A
MAX1557
10
D1
9
INP
8
LX
7
PGND
6
D2
SHDN
GND
TDFN
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1556/MAX1556A/MAX1557
General Description
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
ABSOLUTE MAXIMUM RATINGS
IN, INP, OUT, D2, SHDN to GND ..........................-0.3V to +6.0V
SS, D1 to GND .............................................-0.3V to (VIN + 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
LX Current (Note 1)...........................................................±2.25A
Output Short-Circuit Duration.....................................Continuous
Continuous Power Dissipation (TA = +70°C)
10-Pin TDFN (derate 24.4mW/°C above +70°C) .......1951mW
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
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
(VIN = VINP = VSHDN = 3.6V, TA = - 40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
CONDITIONS
Input Voltage
Undervoltage-Lockout Threshold
Quiescent Supply Current
Shutdown Supply Current
VIN rising and falling, 35mV hysteresis (typ)
V
2.55
25
Dropout
27
42
TA = +25°C
0.1
1
TA = +85°C
0.1
0.75
-0.25
+0.75
+1.75
300mA load
-0.75
0
+0.75
600mA load
-1.5
-0.75
0
1200mA load, MAX1556
-2.75
-2.25
-1.25
1200mA load, MAX1556A
TA = -40°C to +85°C
(Note 2)
-2.25
-0.75
-1.5
+1.5
-2.25
+0.50
1200mA load, MAX1556
-4.0
-1.0
1200
600
V
mA
TA = +25°C
0.01
TA = +85°C
0.01
For preset output voltages
µA
%
600mA load
MAX1557
µA
+2.25
300mA load
MAX1556/MAX1556A
D1 = D2 = GND
MAX1556/MAX1557
VIN
No load
No load
2
V
16
Output Accuracy
FB Threshold Accuracy
UNITS
5.5
2.35
SHDN = GND
2.20
MAX
No switching, D1 = D2 = GND
TA = 0°C to +85°C
(Note 2)
OUT Bias Current
TYP
2.6
Output Voltage Range
Maximum Output Current
MIN
0.1
µA
3
4.5
D1 = D2 = GND,
VOUT = 0.75V at
300mA (typ),
TA = 0°C to +85°C
MAX1556/MAX1557
No load
-0.50
+0.75
+1.75
300mA load
-1.2
0
+1.2
600mA load
-1.75
-0.75
+0.25
1200mA load, MAX1556 only
-3.25
-2.25
-1.25
D1 = D2 = GND,
VOUT = 0.75V
at 300mA (typ),
TA = -40°C to +85°C
MAX1556/MAX1557
No load
-1.25
+2.25
300mA load
-1.75
+1.50
600mA load
-2.75
+0.25
1200mA load, MAX1556 only
-4.25
-1.00
_______________________________________________________________________________________
%
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
(VIN = VINP = VSHDN = 3.6V, TA = - 40°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
CONDITIONS
MAX1556,
D1 = IN, D2 = GND;
MAX1556A
D1 = D2 = IN
Line Regulation
MAX1557,
D1 = IN, D2 = GND
MAX1556/MAX1556A
p-Channel On-Resistance
MAX1557
n-Channel On-Resistance
p-Channel Current-Limit
Threshold
LX Leakage Current
TYP
VIN = 2.6V to 3.6V
-0.37
VIN = 3.6V to 5.5V
0.33
VIN = 2.6V to 3.6V
-0.1
VIN = 3.6V to 5.5V
0.09
VIN = 3.6V
0.19
VIN = 2.6V
0.23
VIN = 3.6V
0.35
VIN = 2.6V
MAX
0.27
VIN = 2.6V
0.33
%
0.35
0.7
0.48
MAX1556/MAX1556A
1.5
1.8
2.1
MAX1557
0.8
1.0
1.2
20
35
45
MAX1556/MAX1556A
1.8
MAX1557
1.0
VIN = 5.5V, LX =
GND or IN
TA = +25°C
0.1
TA = +85°C
0.1
Maximum Duty Cycle
10
100
SS Output Impedance
∆VSS / ISS for ISS = 2µA
SS Discharge Resistance
SHDN = GND, 1mA sink current
Ω
Ω
A
mA
ARMS
µA
%
Minimum Duty Cycle
Internal Oscillator Frequency
UNITS
0.42
VIN = 3.6V
n-Channel Zero Crossing
Threshold
RMS LX Output Current
MIN
0
%
1.1
MHz
kΩ
0.9
1
130
200
300
90
200
Ω
Thermal-Shutdown Threshold
+160
°C
Thermal-Shutdown Hysteresis
15
°C
LOGIC INPUTS (D1, D2, SHDN)
Input-Voltage High
2.6V ≤ VIN ≤ 5.5V
1.4
V
Input-Voltage Low
Input Leakage
0.4
TA = +25°C
0.1
TA = +85°C
0.1
1
V
µA
Note 1: All units are 100% production tested at TA = +25°C. Limits over the operating range are guaranteed by design.
Note 2: For the MAX1556, 3.3V output accuracy is specified with a 4.2V input.
_______________________________________________________________________________________
3
MAX1556/MAX1556A/MAX1557
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.)
EFFICIENCY (%)
70
60
80
70
40
VIN = 2.6V
60
1
10
100
1000
70
VIN = 5V
VIN = 3.6V
VIN = 3V
60
1
10
100
1000
0.1
10,000
1
10
10,000
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
70
VIN = 3V
VIN = 2.6V
1.81
1.80
1.79
TA = +25°C
1.78
1.77
TA = +85°C
1.76
50
1
10
200
TA = -40°C
1.809
TA = +25°C
1.808
1.807
1.806
TA = +85°C
1.805
600
800
1000
1200
TA = +25°C
1.783
1.782
TA = +85°C
2.5
3.0
3.5
4.0
MAX1556/7 toc09
ILOAD = 750mA
VOUT
AC-COUPLED
10mV/div
14
12
VLX
2V/div
0
10
8
6
2
1.803
0
4.0
4.5
INPUT VOLTAGE (V)
5.0
5.5
5.5
HEAVY-LOAD SWITCHING WAVEFORMS
16
1.804
3.5
5.0
SUPPLY CURRENT vs. INPUT VOLTAGE
ILX
4
3.0
4.5
INPUT VOLTAGE (V)
18
SUPPLY CURRENT (µA)
1.810
TA = -40°C
1.784
MAX1556/7 toc08
1.811
400
20
MAX1556/7 toc07
1.812
1.785
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT VOLTAGE vs. INPUT VOLTAGE
WITH NO LOAD
1.786
1.779
0
1000
100
1.787
1.780
1.74
0.1
1.788
1.781
1.75
40
1.789
OUTPUT VOLTAGE (V)
VIN = 5V
TA = -45°C
1.82
OUTPUT VOLTAGE (V)
80
VIN = 3.6V
1.83
MAX1556/7 toc06
1.84
MAX1556/7 toc04
90
2.5
1000
100
LOAD CURRENT (mA)
100
60
VIN = 2.6V
40
0.1
10,000
80
50
40
0.1
4
90
50
50
EFFICIENCY (%)
VIN = 5V
VIN = 3.6V
VIN = 3V
MAX1556/7 toc05
EFFICIENCY (%)
VIN = 3.6V
90
EFFICIENCY (%)
VIN = 5V
100
MAX1556/7 toc02
90
80
100
MAX1556/7 toc01
100
VIN = 4.2V
EFFICIENCY vs. LOAD CURRENT
WITH 1.8V OUTPUT
EFFICIENCY vs. LOAD CURRENT
WITH 2.5V OUTPUT
MAX1556/7 toc03
EFFICIENCY vs. LOAD CURRENT
WITH 3.3V OUTPUT
OUTPUT VOLTAGE (V)
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
500mA/div
0
1
2
3
4
5
6
400ns
INPUT VOLTAGE (V)
_______________________________________________________________________________________
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
EXTERNAL FEEDBACK
SWITCHING WAVEFORMS
LIGHT-LOAD SWITCHING WAVEFORMS
SOFT-START/SHUTDOWN WAVEFORMS
MAX1556/7 toc10b
MAX1556/7 toc10
MAX1556/7 toc11
VOUT
VLX
VOUT
2V/div
0
200mA/div
ILX
500mA/div
0
IIN
500mA/div
0
200mA/div
ILX
0
0
ILX
VIN = 5V, VOUT = 3.3V, IOUT = 500mA
2µs/div
4µs/div
100µs/div
LOAD TRANSIENT
SOFT-START RAMP TIME vs. CSS
MAX1556/7 toc13
MAX1556/7 toc12
10
SOFT-START RAMP TIME (ms)
1V/div
0
CSS = 470pF
RLOAD = 4Ω
0
2V/div
VLX
5V/div
0
VSHDN
20mV/div
AC-COUPLED
50mV/div
AC-COUPLED
VOUT
1
500mA/div
0
IOUT
IOUTMIN = 20mA
0.1
0
500
1000
1500
2000
20µs/div
2500
CSS (pF)
BODE PLOT
LINE TRANSIENT
MAX1556/7 toc14
MAX1556/7 toc15
4V
3.5V
10mV/div
AC-COUPLED
VOUT
500mA/div
0
IOUT
200mA/div
ILX
0
40µs/div
240
30
210
20
180
10
150
0
120
90
-10
0dB
-20
60
PHASE MARGIN = 53°
-30
30
-40
0
-50
IOUTMIN = 180mA
20µs/div
GAIN (dB)
50mV/div
AC-COUPLED
VOUT
VIN
MAX1556/7 toc16
40
PHASE (DEGREES)
LOAD TRANSIENT
-30
COUT = 22µF, RLOAD = 4Ω
-60
0.1
1
10
100
-60
1000
FREQUENCY (kHz)
_______________________________________________________________________________________
5
MAX1556/MAX1556A/MAX1557
Typical Operating Characteristics (continued)
(VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25°C, unless otherwise noted.)
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
Pin Description
PIN
NAME
1
IN
2
GND
FUNCTION
Supply Voltage Input. Connect to a 2.6V to 5.5V source.
Ground. Connect to PGND.
Soft-Start Control. Connect a 1000pF capacitor (CSS) from SS to GND to eliminate input-current
overshoot during startup. CSS is required for normal operation of the MAX1556/MAX1557. For greater
than 22µF total output capacitance, increase CSS to COUT / 22,000 for soft-start. SS is internally
discharged through 200Ω to GND in shutdown.
3
SS
4
OUT
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.
5
SHDN
Shutdown Input. Drive SHDN low to enable low-power shutdown mode. Drive high or connect to IN
for normal operation.
6
D2
7
PGND
OUT Voltage-Select Input. See Table 1.
8
LX
Inductor Connection. Connected to the drains of the internal power MOSFETs. High impedance in
shutdown mode.
9
INP
Supply Voltage, High-Current Input. Connect to a 2.6V to 5.5V source. Bypass with a 10µF ceramic
capacitor to PGND.
10
D1
OUT Voltage-Select Input. See Table 1.
—
EP
Exposed Paddle. Connect to ground plane. EP also functions as a heatsink. Solder to circuit-board
ground plane to maximize thermal dissipation.
Power Ground. Connect to GND.
Table 1. Output-Voltage-Select Truth Table
D2
MAX1556
VOUT
0
MAX1556A
VOUT
MAX1557
VOUT
0
Adjustable
(VFB = 0.75)
from 0.75V to
VIN
3.3V
Adjustable
(VFB = 0.75)
from 0.75V to
VIN
0
1
3.3V
1.5V
1.5V
1
0
2.5V
1.2V
1.3V
1
1
1.8V
2.5V
1.0V
D1
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 converters deliver a guaranteed 1.2A/600mA at output voltages from 0.75V to V IN . They use a 1MHz PWM
current-mode control scheme with internal compensation,
allowing for tiny external components and a fast transient
response. At light loads the MAX1556/MAX1557 automatically switch to pulse-skipping mode to keep the quiescent supply current as low as 16µA. Figures 2 and 3
show the typical application circuits.
6
Control Scheme
During PWM operation the converters use a fixed-frequency, current-mode control scheme. The heart of the
current-mode PWM controller is an open-loop, multipleinput comparator that compares the error-amp voltage
feedback signal against the sum of the amplified current-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 current limit, the high-side MOSFET is turned off and the
low-side MOSFET remains on until the next clock cycle.
_______________________________________________________________________________________
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
BIAS
CURRENT-LIMIT
COMPARATOR
VCS
CURRENT
SENSE
MAX1556/MAX1556A/MAX1557
IN
SHDN
SHORT-CIRCUIT
PROTECTION
CLOCK
1MHz
0.675V
PWM
COMPARATOR
INP
PWM
AUTO SKIP
CONTROL
LX
SLOPE
COMP
PGND
SKIP-OVER
ENTER SKIP/
SR OFF
ZERO-CROSS
DETECT
ERROR
AMPLIFIER
OUT
REFERENCE
1.25V
GND
D1
OUTPUT
VOLTAGE
SELECTOR
D2
MAX1556
MAX1556A
MAX1557
SS
Figure 1. Functional Diagram
L1
3.3µH
INPUT
2.6V TO 5.5V
INP
R1
100Ω
C4
0.47µF
C1
10µF
LX
OUTPUT
0.75V TO VIN
1.2A
INP
C4
10µF
C2
22µF
MAX1556
MAX1556A
IN
L2
4.7µH
INPUT
2.6V TO 5.5V
VOLTAGE
SELECT
OUT
SS
ON
OFF
SHDN
GND
Figure 2. MAX1556 Typical Application Circuit
MAX1557
PGND
D1
D2
C5
22µF
IN
PGND
VOLTAGE
SELECT
LX
OUTPUT
0.75V TO VIN
600mA
C3
1000pF
D1
OUT
D2
SS
ON
OFF
SHDN
C6
1000pF
GND
Figure 3. MAX1557 Typical Application Circuit
_______________________________________________________________________________________
7
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
from light load to full load. Additionally, the MAX1556,
MAX1556A, and MAX1557 use a wide-bandwidth feedback loop to respond more quickly to a load transient
than regulators using conventional integrating feedback
loops (see Load Transient in the Typical Operating
Characteristics).
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 maintain 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.
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.
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
8
1.0
CHANGE IN OUTPUT VOLTAGE (%)
As the load current decreases, the converters enter a
pulse-skip mode in which the PWM comparator is disabled. 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.
0.5
0
VIN = 3.6V
VIN = 5.5V
-0.5
-1.0
VIN = 2.6V
-1.5
-2.0
-2.5
0
200
400
600
800
1000
1200
LOAD CURRENT (mA)
Figure 4. MAX1556 Voltage-Positioning Load Line
1.0
0.8
CHANGE IN OUTPUT VOLTAGE (%)
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
0.6
0.4
VIN = 3.6V
0.2
VIN = 5.5V
0
-0.2
VIN = 2.6V
-0.4
-0.6
-0.8
-1.0
0
200
400
600
LOAD CURRENT (mA)
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 system 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 regulation voltage. The quiescent current in this state rises
minimally to only 27µA (typ), which aids in extending battery life. This dropout/100% duty-cycle operation achieves
long battery life by taking full advantage of the entire battery range.
The input voltage required to maintain regulation is a
function of the output voltage and the load. The difference between this minimum input voltage and the output voltage is called the dropout voltage. The dropout
voltage is therefore a function of the on-resistance of
the internal p-channel MOSFET (R DS(ON)P) and the
inductor resistance (DCR).
_______________________________________________________________________________________
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
MANUFACTURER
PART
VALUE (µH)
DCR (mΩ)
ISAT (mA)
SIZE (mm)
SHIELDED
Taiyo Yuden
LMNP04SB3R3N
3.3
36
1300
5 x 5 x 2.0
Yes
Taiyo Yuden
LMNP04SB4R7N
4.7
50
1200
5 x 5 x 2.0
Yes
TOKO
D52LC
3.5
73
1340
5 x 5 x 2.0
Yes
TOKO
D52LC
4.7
87
1140
5 x 5 x 2.0
Yes
Sumida
CDRH3D16
4.7
50
1200
3.8 x 3.8 x 1.8
Yes
TOKO
D412F
4.7
100*
1200*
4.8 x 4.8 x 1.2
Yes
Murata
LQH32CN
4.7
97
790
2.5 x 3.2 x 2.0
No
Sumitomo
CXL180
4.7
70*
1000*
3.0 x 3.2 x 1.7
No
Sumitomo
CXLD140
4.7
100*
800*
2.8 x 3.2 x 1.5
No
*Estimated based upon similar-valued prototype inductors.
VDROPOUT = IOUT x (RDS(ON)P + DCR)
RDS(ON)P is given in the Electrical Characteristics. DCR
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 transients at the input source. Soft-start is particularly useful 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:
CSS =
COUT
22000
Thermal Shutdown
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 synchronous rectifier are turned off. The device resumes normal operation when the junction temperature falls
below +145°C.
Applications Information
The MAX1556/MAX1556A/MAX1557 are optimized for
use with small external components. The correct selection of inductors and input and output capacitors
ensures high efficiency, low output ripple, and fast transient response.
Adjusting the Output Voltage
Soft-start is implemented by exponentially ramping up
the output voltage from 0 to VOUT(NOM) with a time constant equal to C SS times 200kΩ (see the Typical
Operating Characteristics). Assuming three time constants to full output voltage, use the following formula to
calculate the soft-start time:
t SS = 600 x 103 x CSS
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 control 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 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 waveform 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.
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 regulation 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
MAX1556/MAX1556A/MAX1557
Table 2. Inductor Selection
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
can be reduced by applying a correction factor to the
feedback resistor equation.
OUTPUT
First, calculate the correction factor, k, by plugging the
desired output voltage into the following formula:
k = 1.06 x 10
−2
⎛V
− 0.75V ⎞
V x ⎜ OUTPUT
⎟
3.6V
⎝
⎠
R2
ERROR
AMPLIFIER
OUT
R3
REFERENCE
1.25V
k represents the shift in the operating point at the feedback node (OUT).
Select the lower feedback resistor, R3, to be ≤35.7kΩ to
ensure stability and solve for R2:
⎛ 0.75V − k ⎞
⎜V
⎟ =
⎝ OUTPUT ⎠
R3
(R3 + R2)
Inductor Selection
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 application with 1.2A full load, use a 3.3µH inductor with at
least 1.34A saturation current. For lower full-load currents 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.
Capacitor Selection
Ceramic input and output capacitors are recommended for most applications. For best stability over a wide
temperature range, use capacitors with an X5R or better dielectric due to their small size, low ESR, and low
temperature coefficients.
Output Capacitor
The output capacitor COUT is required to keep the output voltage ripple small and to ensure regulation loop
stability. COUT must have low impedance at the switching frequency. A 22µF ceramic output capacitor is recommended 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.
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
CINP should be kept very low at the switching frequency. A minimum value of 10µF is recommended at INP for
10
SS
Figure 6. Adjustable Output Voltage
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.
Soft-Start Capacitor
The soft-start capacitor, CSS, is required for proper
operation of the MAX1556/MAX1556A/MAX1557. The
recommended value of CSS is discussed in the SoftStart section. Soft-start times for various soft-start
capacitors are shown in the Typical Operating
Characteristics.
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.
______________________________________________________________________________________
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
PROCESS: BiCMOS
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 drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
10 TDFN
T1033-1
21-0137
90-0003
______________________________________________________________________________________
11
MAX1556/MAX1556A/MAX1557
Chip Information
MAX1556/MAX1556A/MAX1557
16µA IQ, 1.2A PWM DC-DC
Step-Down Converters
Revision History
REVISION
NUMBER
REVISION
DATE
DESCRIPTION
0
7/04
Initial release
1
3/08
Adding MAX1556A as a new version
2
6/10
Added soldering temperature, added TOC for external feedback switching
waveforms, and added paragraph discussing noise coupling when using
external feedback resistors
PAGES
CHANGED
—
1–12
1, 2, 5, 6, 9,
10, 11
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.