MAX1556ETB-T [MAXIM]
Switching Regulator, Current-mode, 2.25A, 1100kHz Switching Freq-Max, BICMOS, 3 X 3 MM, 0.80 MM HEIGHT, MO-229WEED-3, TDFN-10;
| 型号： | MAX1556ETB-T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Switching Regulator, Current-mode, 2.25A, 1100kHz Switching Freq-Max, BICMOS, 3 X 3 MM, 0.80 MM HEIGHT, MO-229WEED-3, TDFN-10 信息通信管理 开关 |
| 文件： | 总12页 (文件大小：714K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3336; Rev 0; 7/04
16µA I , 1.2A PWM
Q
Step-Down DC-DC Converters
General Description
Features
The MAX1556/MAX1557 are low-operating-current
(16µA), fixed-frequency step-down regulators. High effi-
ciency, 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.
♦ Up to 97% Efficiency
♦ 95% Efficiency at 1mA Load Current
♦ Low 16µA Quiescent Current
♦ 1MHz PWM Switching
♦ Tiny 3.3µH Inductor
♦ Selectable 3.3V, 2.5V, 1.8V, 1.5V, 1.3V, 1.0V, and
Adjustable Output
♦ 1.2A Guaranteed Output Current (MAX1556)
The MAX1556/MAX1557 contain a low-on-resistance
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 fixed-frequency
operation while still providing excellent efficiency at
both light and full loads. A 1MHz PWM switching fre-
quency keeps components small. Both devices also
feature an adjustable soft-start to minimize battery tran-
sient loading.
♦ Voltage Positioning Optimizes Load-Transient
Response
♦ Low 27µA Quiescent Current in Dropout
♦ Low 0.1µA Shutdown Current
♦ No External Schottky Diode Required
♦ Analog Soft-Start with Zero Overshoot Current
♦ Small, 10-Pin, 3mm x 3mm TDFN Package
The MAX1556/MAX1557 are available in a tiny 10-pin
TDFN (3mm x 3mm) package.
Applications
Ordering Information
TOP
MARK
PDAs and Palmtop Computers
Cell Phones and Smart Phones
Digital Cameras and Camcorders
Portable MP3 and DVD Players
Hand-Held Instruments
PART
TEMP RANGE PIN-PACKAGE
10 TDFN-EP*
(T1033-1)
MAX1556ETB -40°C to +85°C
ACQ
10 TDFN-EP*
(T1033-1)
MAX1557ETB -40°C to +85°C
*EP = Exposed paddle.
ACR
Typical Operating Circuit
Pin Configuration
TOP VIEW
OUTPUT
0.75V TO V
INPUT
2.6V TO 5.5V
IN
INP
LX
1
2
3
4
5
IN
10 D1
MAX1556/
MAX1557
GND
SS
INP
LX
9
8
7
6
PGND
MAX1556/
MAX1557
IN
D1
D2
PGND
D2
OUT
SHDN
VOLTAGE
SELECT
OUT
SS
ON
OFF
SHDN
TDFN
GND
________________________________________________________________ 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.
16µA I , 1.2A PWM DC-DC
Q
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 (V + 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
LX Current (Note 1)........................................................... 2.25A
Output Short-Circuit Duration.....................................Continuous
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
IN
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
(V = V
IN
= V
= 3.6V, T = - 40°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.) (Note 1)
SHDN A A
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
(Note 2)
600mA load
-0.75
-2.25
1200mA load, MAX1556 only
No load
-2.75
-0.75
-1.5
-1.25
+2.25
+1.5
+0.50
-1.0
Output Accuracy
%
300mA load
T
A
= -40°C to +85°C
(Note 2)
600mA load
-2.25
-4.0
1200mA load, MAX1556 only
MAX1556
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
A
For preset output voltages
No load
4.5
-0.50
-1.2
+0.75
0
+1.75
+1.2
D1 = D2 = GND,
= 0.75V at
300mA load
V
OUT
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
1200mA load, MAX1556 only
No load
FB Threshold Accuracy
%
D1 = D2 = GND,
= 0.75V
300mA load
V
OUT
at 300mA (typ),
= -40°C to +85°C
600mA load
T
A
1200mA load, MAX1556 only
2
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(V = V
IN
= V
= 3.6V, T = - 40°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.) (Note 1)
SHDN A A
INP
PARAMETER
CONDITIONS
MIN
TYP
-0.37
0.33
-0.1
0.09
0.19
0.23
0.35
0.42
0.27
0.33
1.8
MAX
UNITS
V
V
V
V
V
V
V
V
= 2.6V to 3.6V
= 3.6V to 5.5V
= 2.6V to 3.6V
= 3.6V to 5.5V
= 3.6V
IN
IN
IN
IN
IN
IN
IN
IN
MAX1556,
D1 = IN, D2 = GND
Line Regulation
%
MAX1557,
D1 = IN, D2 = GND
0.35
0.7
MAX1556
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
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
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
Thermal-Shutdown Hysteresis
SHDN = GND, 1mA sink current
+160
15
°C
°C
LOGIC INPUTS (D1, D2, SHDN)
Input-Voltage High
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
100
90
80
70
60
50
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
T
= -45°C
A
V
= 5V
IN
V
= 3.6V
IN
T
= -40°C
A
T
= +25°C
V
= 3V
T
= +25°C
A
A
IN
V
= 2.6V
IN
T
= +85°C
A
T
= +85°C
A
40
0.1
1
10
100
1000
0
200
400
600
800 1000 1200
2.5
3.0
3.5
4.0
4.5
5.0
5.5
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
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
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.)
INP
A
LIGHT-LOAD SWITCHING WAVEFORMS
SOFT-START/SHUTDOWN WAVEFORMS
SOFT-START RAMP TIME vs. C
SS
MAX1556/7 toc10
MAX1556/7 toc11
10
5V/div
0
V
SHDN
20mV/div
V
OUT
AC-COUPLED
V
1V/div
0
OUT
C
R
= 470pF
SS
= 4Ω
LOAD
V
2V/div
1
LX
500mA/div
0
I
LX
0
200mA/div
0
500mA/div
0
I
I
IN
LX
0.1
4µs/div
100µs/div
0
500
1000
1500
(pF)
2000
2500
C
SS
LOAD TRANSIENT
LOAD TRANSIENT
MAX1556/7 toc14
MAX1556/7 toc13
50mV/div
AC-COUPLED
50mV/div
AC-COUPLED
V
V
OUT
OUT
500mA/div
0
500mA/div
0
I
I
OUT
OUT
I
= 20mA
I
= 180mA
OUTMIN
OUTMIN
20µs/div
20µs/div
LINE TRANSIENT
BODE PLOT
MAX1556/7 toc15
MAX1556/7 toc16
40
30
20
10
0
240
210
180
150
120
90
4V
V
IN
3.5V
-10
-20
-30
-40
-50
-60
10mV/div
AC-COUPLED
V
OUT
0dB
60
PHASE MARGIN = 53°
30
200mA/div
0
0
I
LX
-30
C
= 22µF, R
= 4Ω
LOAD
OUT
-60
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
—
10
EP
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
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.
Table 1. Output-Voltage-Select Truth Table
D1
0
D2
0
MAX1556 V
MAX1557 V
OUT
OUT
Adjustable from 0.75V to V
IN
0
1
3.3V
2.5V
1.8V
1.5V
1
0
1.3V
1.0V
1
1
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
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
MAX1557
SS
Figure 1. Functional Diagram
OUTPUT
OUTPUT
L2
4.7µH
0.75V TO V
L1
3.3µH
INPUT
2.6V TO 5.5V
IN
0.75V TO V
INPUT
2.6V TO 5.5V
IN
600mA
1.2A
INP
LX
INP
IN
LX
C4
10µF
C1
10µF
C5
22µF
R1
100Ω
C2
22µF
MAX1557
MAX1556
PGND
PGND
IN
C4
D1
D2
0.47µF
VOLTAGE
SELECT
D1
D2
OUT
SS
VOLTAGE
SELECT
OUT
SS
ON
C6
1000pF
ON
C3
1000pF
OFF
SHDN
OFF
SHDN
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
Load-Transient Response/
Voltage Positioning
V
= 2.6V
IN
The MAX1556/MAX1557 match the load regulation to
the voltage droop seen during transients. This is some-
times 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 min-
imal voltage drop during a transition from light load to
400
600
800 1000
0
200
1200
LOAD CURRENT (mA)
Figure 4. MAX1556 Voltage-Positioning Load Line
full load. Additionally, the MAX1556 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).
1.0
0.8
0.6
0.4
The MAX1556/MAX1557 use of a wide-band control
loop and voltage positioning allows superior load-tran-
sient response by minimizing the amplitude and dura-
tion 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.
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
0
200
400
600
LOAD CURRENT (mA)
Voltage positioning on the MAX1556/MAX1557 allows
up to 2.25% (typ) of load-regulation voltage shift but
has no further transient droop. Thus, during load tran-
sients, 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 oscil-
loscope (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
operate normally down to 3V or less. The MAX1556/
MAX1557 allow the output to follow the input battery
voltage as it drops below the regulation voltage. The qui-
escent 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 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/MAX1557 function with a low input-to-out-
put voltage difference by operating at 100% duty cycle.
In this state, the high-side p-channel MOSFET is always
on. This is particularly useful in battery-powered appli-
cations with a 3.3V output. The system and load might
the internal p-channel MOSFET (R
inductor resistance (DCR).
) and the
DS(ON)P
V
= I
x (R
+ DCR)
DS(ON)P
DROPOUT
OUT
8
_______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
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.
(R
) is given in the Electrical Characteristics. DCR
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.
DS(ON)P
for a few recommended inductors is listed in Table 2.
Soft-Start
The MAX1556/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:
Applications Information
The MAX1556/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
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
can be reduced by applying a correction factor to the
feedback resistor equation.
C
22000
OUT
C
=
SS
Soft-start is implemented by exponentially ramping up
the output voltage from 0 to V with a time con-
OUT(NOM)
times 200kΩ (see the Typical
Operating Characteristics). Assuming three time con-
stants to full output voltage, use the following formula to
calculate the soft-start time:
stant equal to C
SS
First, calculate the correction factor, k, by plugging the
desired output voltage into the following formula:
3
t
= 600 x 10 x C
V
− 0.75V
3.6V
SS
SS
−2
OUTPUT
k = 1.06 x 10 V x
Shutdown Mode
k represents the shift in the operating point at the feed-
back node (OUT).
Connecting SHDN to GND or logic low places the
MAX1556/MAX1557 in shutdown mode and reduces
supply current to 0.1µA. In shutdown, the control cir-
cuitry 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.
Select the lower feedback resistor, R3, to be ≤35.7kΩ to
ensure stability and solve for R2:
0.75V −k
R3
R3+R2
=
V
Thermal Shutdown
As soon as the junction temperature of the
MAX1556/MAX1557 exceeds +160°C, the ICs go into
OUTPUT
_______________________________________________________________________________________
9
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
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 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 dif-
fers among different manufacturers and inductor types
and has an impact on the efficiency. See Table 2 for
recommended inductors and manufacturers.
OUTPUT
R2
ERROR
AMPLIFIER
OUT
R3
REFERENCE
1.25V
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.
SS
Output Capacitor
is required to keep the out-
Figure 6. Adjustable Output Voltage
The output capacitor C
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
PC Board Layout and Routing
Due to fast-switching waveforms and high-current
paths, careful PC board layout is required. An evalua-
tion 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 to the IC as possible.
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
C
should be kept very low at the switching frequen-
INP
cy. A minimum value of 10µF is recommended at INP for
most applications. The input capacitor can be increased
for better input filtering.
Chip Information
TRANSISTOR COUNT: 7567
IN Input Filter
In all MAX1557 applications, connect INP directly to IN
and bypass INP as described in the Input Capacitor sec-
tion. No additional bypass capacitor is required at IN.
For applications using the MAX1556, 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.
PROCESS: BiCMOS
Soft-Start Capacitor
The soft-start capacitor, C , is required for proper
SS
operation of the MAX1556/MAX1557. The recommend-
ed value of C is discussed in the Soft-Start section.
SS
Soft-start times for various soft-start capacitors are
shown in the Typical Operating Characteristics.
10 ______________________________________________________________________________________
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.
D
N
PIN 1
INDEX
AREA
E
E2
DETAIL A
C
C
L
L
L
L
A
e
e
PACKAGE OUTLINE, 6, 8, 10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
1
NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY
21-0137
F
2
______________________________________________________________________________________ 11
16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.
COMMON DIMENSIONS
SYMBOL
MIN.
0.70
2.90
2.90
0.00
0.20
MAX.
0.80
3.10
3.10
0.05
0.40
A
D
E
A1
L
k
0.25 MIN.
0.20 REF.
A2
PACKAGE VARIATIONS
PKG. CODE
T633-1
N
6
D2
E2
e
JEDEC SPEC
MO229 / WEEA
MO229 / WEEC
b
[(N/2)-1] x e
1.90 REF
1.95 REF
2.00 REF
2.40 REF
2.40 REF
1.50 0.10 2.30 0.10 0.95 BSC
1.50 0.10 2.30 0.10 0.65 BSC
0.40 0.05
0.30 0.05
T833-1
8
T1033-1
T1433-1
T1433-2
10
14
14
1.50 0.10 2.30 0.10 0.50 BSC MO229 / WEED-3 0.25 0.05
1.70 0.10 2.30 0.10 0.40 BSC
1.70 0.10 2.30 0.10 0.40 BSC
- - - -
- - - -
0.20 0.03
0.20 0.03
PACKAGE OUTLINE, 6, 8, 10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
2
21-0137
F
2
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
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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