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19-3336; Rev 2; 6/10
KIT ATION EVALU ABLE AVAIL
16A IQ, 1.2A PWM Step-Down DC-DC Converters
General Description Features
o Up to 97% Efficiency o 95% Efficiency at 1mA Load Current o Low 16A Quiescent Current o 1MHz PWM Switching o Tiny 3.3H 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 27A Quiescent Current in Dropout Low 0.1A Shutdown Current No External Schottky Diode Required Analog Soft-Start with Zero Overshoot Current
MAX1556/MAX1556A/MAX1557
The MAX1556/MAX1556A/MAX1557 are low-operatingcurrent (16A), fixed-frequency step-down regulators. High efficiency, low-quiescent operating current, low dropout, and minimal (27A) 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.
o Small, 10-Pin, 3mm x 3mm TDFN Package
Ordering Information
PART MAX1556ETB+ MAX1556AETB+ MAX1557ETB+ TEMP RANGE PIN-PACKAGE TOP MARK ACQ AUJ ACR
Applications
PDAs and Palmtop Computers Cell Phones and Smart Phones Digital Cameras and Camcorders Portable MP3 and DVD Players Hand-Held Instruments
-40C to +85C 10 TDFN-EP* -40C to +85C 10 TDFN-EP* -40C to +85C 10 TDFN-EP*
*EP = Exposed paddle. +Denotes a lead(Pb)-free/RoHS-compliant package.
Typical Operating Circuit
INPUT 2.6V TO 5.5V OUTPUT 0.75V TO VIN INP LX
TOP VIEW
Pin Configuration
IN VOLTAGE SELECT ON OFF D1 D2
MAX1556 MAX1556A MAX1557
PGND OUT SS
IN
GND SS OUT SHDN
1 2 3 4 5
TDFN
10 9
D1
INP LX
MAX1556 MAX1556A MAX1557
8 7 6
PGND D2
SHDN GND
________________________________________________________________ Maxim Integrated Products
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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.
16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
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 = +70C) 10-Pin TDFN (derate 24.4mW/C above +70C) .......1951mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C Soldering Temperature (reflow) .......................................+260C
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 = - 40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Undervoltage-Lockout Threshold Quiescent Supply Current Shutdown Supply Current Output Voltage Range No load 300mA load TA = 0C to +85C (Note 2) Output Accuracy TA = -40C to +85C (Note 2) MAX1556/MAX1556A MAX1557 D1 = D2 = GND MAX1556/MAX1557 D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = 0C to +85C MAX1556/MAX1557 D1 = D2 = GND, VOUT = 0.75V at 300mA (typ), TA = -40C to +85C MAX1556/MAX1557 TA = +25C TA = +85C No load 300mA load 600mA load 1200mA load, MAX1556 only No load 300mA load 600mA load 1200mA load, MAX1556 only -0.50 -1.2 -1.75 -3.25 -1.25 -1.75 -2.75 -4.25 600mA load 1200mA load, MAX1556 1200mA load, MAX1556A No load 300mA load 600mA load 1200mA load, MAX1556 Maximum Output Current -0.75 -1.5 -2.25 -4.0 1200 600 0.01 0.01 3 +0.75 0 -0.75 -2.25 4.5 +1.75 +1.2 +0.25 -1.25 +2.25 +1.50 +0.25 -1.00 % 0.1 A VIN rising and falling, 35mV hysteresis (typ) No switching, D1 = D2 = GND Dropout SHDN = GND TA = +25C TA = +85C 0.75 -0.25 -0.75 -1.5 -2.75 +0.75 0 -0.75 -2.25 -2.25 +2.25 +1.5 +0.50 -1.0 mA CONDITIONS MIN 2.6 2.20 2.35 16 27 0.1 0.1 VIN +1.75 +0.75 0 -1.25 % TYP MAX 5.5 2.55 25 42 1 UNITS V V A A V
OUT Bias Current
For preset output voltages
FB Threshold Accuracy
2
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16A IQ, 1.2A PWM DC-DC Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VINP = VSHDN = 3.6V, TA = - 40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER MAX1556, D1 = IN, D2 = GND; MAX1556A D1 = D2 = IN MAX1557, D1 = IN, D2 = GND MAX1556/MAX1556A p-Channel On-Resistance MAX1557 n-Channel On-Resistance p-Channel Current-Limit Threshold n-Channel Zero Crossing Threshold RMS LX Output Current LX Leakage Current Maximum Duty Cycle Minimum Duty Cycle Internal Oscillator Frequency SS Output Impedance SS Discharge Resistance Thermal-Shutdown Threshold Thermal-Shutdown Hysteresis LOGIC INPUTS (D1, D2, SHDN) Input-Voltage High Input-Voltage Low Input Leakage TA = +25C TA = +85C 0.1 0.1 2.6V VIN 5.5V 1.4 0.4 1 V V A VSS / ISS for ISS = 2A SHDN = GND, 1mA sink current 0.9 130 1 200 90 +160 15 MAX1556/MAX1556A MAX1557 VIN = 5.5V, LX = GND or IN TA = +25C TA = +85C 100 0 1.1 300 200 0.1 0.1 VIN = 3.6V VIN = 2.6V MAX1556/MAX1556A MAX1557 1.5 0.8 20 CONDITIONS VIN = 2.6V to 3.6V VIN = 3.6V to 5.5V VIN = 2.6V to 3.6V VIN = 3.6V to 5.5V VIN = 3.6V VIN = 2.6V VIN = 3.6V VIN = 2.6V MIN TYP -0.37 0.33 -0.1 0.09 0.19 0.23 0.35 0.42 0.27 0.33 1.8 1.0 35 2.1 1.2 45 1.8 1.0 10 0.48 A mA ARMS A % % MHz k C C 0.7 0.35 MAX UNITS
MAX1556/MAX1556A/MAX1557
Line Regulation
%
Note 1: All units are 100% production tested at TA = +25C. 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.
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
Typical Operating Characteristics
(VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT WITH 3.3V OUTPUT
MAX1556/7 toc01
EFFICIENCY vs. LOAD CURRENT WITH 2.5V OUTPUT
MAX1556/7 toc02
EFFICIENCY vs. LOAD CURRENT WITH 1.8V OUTPUT
MAX1556/7 toc03
100 90 VIN = 4.2V EFFICIENCY (%) 80 70 60 50 40 0.1 1 10 100 1000 VIN = 3.6V VIN = 5V
100 90 EFFICIENCY (%) 80 70 60 50 40 VIN = 5V VIN = 3.6V VIN = 3V VIN = 2.6V
100 90 EFFICIENCY (%) 80 VIN = 2.6V 70 60 50 40 VIN = 5V VIN = 3.6V VIN = 3V
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)
MAX1556/7 toc04
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1556/7 toc05
OUTPUT VOLTAGE vs. INPUT VOLTAGE WITH 600mA LOAD
1.788 1.787 OUTPUT VOLTAGE (V) 1.786 1.785 1.784 1.783 1.782 1.781 1.780 1.779 TA = +85C TA = +25C TA = -40C
MAX1556/7 toc06
100 90 EFFICIENCY (%) 80 VIN = 3.6V 70 60 50 40 0.1 1 10 100 VIN = 3V VIN = 2.6V VIN = 5V
1.84 1.83 1.82 OUTPUT VOLTAGE (V) 1.81 1.80 1.79 1.78 1.77 1.76 1.75 1.74 TA = +85C TA = +25C TA = -45C
1.789
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
MAX1556/7 toc07
SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1556/7 toc08
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1556/7 toc09
1.812 1.811 1.810 OUTPUT VOLTAGE (V) 1.809 1.808 1.807 1.806 1.805 1.804 1.803 2.5 3.0 3.5 4.0 4.5 5.0 TA = +85C TA = -40C TA = +25C
20 18 16 SUPPLY CURRENT (A) 14 12 10 8 6 4 2 0
ILOAD = 750mA VOUT AC-COUPLED 10mV/div
VLX 2V/div 0 ILX 500mA/div 0 1 2 3 4 5 6 400ns INPUT VOLTAGE (V)
5.5
INPUT VOLTAGE (V)
4
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16A IQ, 1.2A PWM DC-DC Step-Down Converters
Typical Operating Characteristics (continued)
(VIN = VINP = 3.6V, D1 = D2 = SHDN = IN, Circuits of Figures 2 and 3, TA = +25C, unless otherwise noted.)
EXTERNAL FEEDBACK SWITCHING WAVEFORMS
MAX1556/7 toc10b
MAX1556/MAX1556A/MAX1557
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1556/7 toc10
SOFT-START/SHUTDOWN WAVEFORMS
MAX1556/7 toc11
VOUT
20mV/div AC-COUPLED VLX 2V/div 0
VSHDN VOUT
5V/div 0 1V/div 0
CSS = 470pF RLOAD = 4
VLX
2V/div 0 200mA/div ILX VIN = 5V, VOUT = 3.3V, IOUT = 500mA 4s/div 2s/div
ILX 200mA/div 0 IIN 100s/div
500mA/div 0
ILX
0
500mA/div 0
SOFT-START RAMP TIME vs. CSS
MAX1556/7 toc12
LOAD TRANSIENT
MAX1556/7 toc13
10
SOFT-START RAMP TIME (ms)
VOUT
50mV/div AC-COUPLED
1
500mA/div IOUT IOUTMIN = 20mA 0
0.1 0 500 1000 1500 2000 2500 20s/div CSS (pF)
LOAD TRANSIENT
MAX1556/7 toc14
LINE TRANSIENT
MAX1556/7 toc15
BODE PLOT
40 4V 30 20 10 GAIN (dB) 0 -10 -20 -30 -40 -50 0 -60 0.1 COUT = 22F, RLOAD = 4 1 10 FREQUENCY (kHz) 100 0dB PHASE MARGIN = 53
MAX1556/7 toc16
240 210 180 PHASE (DEGREES) 150 120 90 60 30 0 -30
VOUT
50mV/div AC-COUPLED
VIN
3.5V
VOUT 500mA/div 0 IOUT IOUTMIN = 180mA 20s/div 40s/div ILX
10mV/div AC-COUPLED 200mA/div
-60 1000
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
Pin Description
PIN 1 2 NAME IN GND 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 22F total output capacitance, increase CSS to COUT / 22,000 for soft-start. SS is internally discharged through 200 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. Shutdown Input. Drive SHDN low to enable low-power shutdown mode. Drive high or connect to IN for normal operation. OUT Voltage-Select Input. See Table 1. Power Ground. Connect to GND. Inductor Connection. Connected to the drains of the internal power MOSFETs. High impedance in shutdown mode. Supply Voltage, High-Current Input. Connect to a 2.6V to 5.5V source. Bypass with a 10F ceramic capacitor to PGND. 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. FUNCTION Supply Voltage Input. Connect to a 2.6V to 5.5V source.
3
SS
4
OUT
5 6 7 8 9 10 --
SHDN D2 PGND LX INP D1 EP
Table 1. Output-Voltage-Select Truth Table
D1 D2 MAX1556 VOUT Adjustable (VFB = 0.75) from 0.75V to VIN 3.3V 2.5V 1.8V MAX1556A VOUT MAX1557 VOUT Adjustable (VFB = 0.75) from 0.75V to VIN 1.5V 1.3V 1.0V
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.
0
0
3.3V
0 1 1
1 0 1
1.5V 1.2V 2.5V
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 16A. Figures 2 and 3 show the typical application circuits.
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
CLOCK 1MHz SHORT-CIRCUIT PROTECTION
IN SHDN BIAS
CURRENT SENSE
VCS
CURRENT-LIMIT COMPARATOR
INP
0.675V PWM COMPARATOR SLOPE COMP
PWM AUTO SKIP CONTROL
LX
PGND SKIP-OVER ENTER SKIP/ SR OFF ZERO-CROSS DETECT
ERROR AMPLIFIER
OUT
REFERENCE 1.25V
GND
OUTPUT VOLTAGE SELECTOR
D1 D2
MAX1556 MAX1556A MAX1557 SS
Figure 1. Functional Diagram
OUTPUT 0.75V TO VIN 1.2A OUTPUT 0.75V TO VIN 600mA
INPUT 2.6V TO 5.5V INP R1 100 C1 10F LX
L1 3.3H
INPUT 2.6V TO 5.5V C4 10F
L2 4.7H INP LX C5 22F
MAX1556 MAX1556A IN
PGND D1 D2 SHDN GND OUT SS
C2 22F
MAX1557
PGND IN
C4 0.47F
VOLTAGE SELECT ON OFF
VOLTAGE SELECT C3 1000pF ON OFF
D1 D2
OUT SS C6 1000pF
SHDN GND
Figure 2. MAX1556 Typical Application Circuit
Figure 3. MAX1557 Typical Application Circuit
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
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 16A. See the Light-Load Switching Waveforms and Load Transient graphs in the Typical Operating Characteristics.
1.0 0.5 0 -0.5 -1.0 VIN = 2.6V -1.5 -2.0 -2.5 0 200 400 600 800 1000 1200 LOAD CURRENT (mA) VIN = 3.6V VIN = 5.5V
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 100s 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.
Figure 4. MAX1556 Voltage-Positioning Load Line
1.0 0.8 CHANGE IN OUTPUT VOLTAGE (%) 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 200 400 600 LOAD CURRENT (mA) VIN = 2.6V VIN = 3.6V VIN = 5.5V
Figure 5. MAX1557 Voltage-Positioning Load Line
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
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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 27A (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).
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16A IQ, 1.2A PWM DC-DC Step-Down Converters
Table 2. Inductor Selection
MANUFACTURER Taiyo Yuden Taiyo Yuden TOKO TOKO Sumida TOKO Murata Sumitomo Sumitomo PART LMNP04SB3R3N LMNP04SB4R7N D52LC D52LC CDRH3D16 D412F LQH32CN CXL180 CXLD140 VALUE (H) 3.3 4.7 3.5 4.7 4.7 4.7 4.7 4.7 4.7 DCR (m) 36 50 73 87 50 100* 97 70* 100* ISAT (mA) 1300 1200 1340 1140 1200 1200* 790 1000* 800* SIZE (mm) 5 x 5 x 2.0 5 x 5 x 2.0 5 x 5 x 2.0 5 x 5 x 2.0 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 SHIELDED Yes Yes Yes Yes Yes Yes No No No
MAX1556/MAX1556A/MAX1557
*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.
Thermal Shutdown
As soon as the junction temperature of the MAX1556/MAX1556A/MAX1557 exceeds +160C, 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 +145C.
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 22F 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
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
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
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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 200k (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 0.1A. 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.
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
can be reduced by applying a correction factor to the feedback resistor equation. First, calculate the correction factor, k, by plugging the desired output voltage into the following formula: k = 1.06 x 10
-2
ERROR AMPLIFIER OUTPUT R2 OUT R3
V - 0.75V V x OUTPUT 3.6V
REFERENCE 1.25V
k represents the shift in the operating point at the feedback node (OUT). Select the lower feedback resistor, R3, to be 35.7k to ensure stability and solve for R2: 0.75V - k V = OUTPUT
(R3 + R2)
Inductor Selection
Figure 6. Adjustable Output Voltage
R3
SS
A 4.7H 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 3.3H 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.
most applications. The input capacitor can be increased for better input filtering.
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.
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 100 resistor between INP and IN, and connect a 0.47F 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.
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 22F 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 10F is recommended at INP for
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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.
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16A IQ, 1.2A PWM DC-DC Step-Down Converters
Chip Information
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 10 TDFN PACKAGE CODE T1033-1 OUTLINE NO. 21-0137 LAND PATTERN NO. 90-0003
MAX1556/MAX1556A/MAX1557
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16A IQ, 1.2A PWM DC-DC Step-Down Converters MAX1556/MAX1556A/MAX1557
Revision History
REVISION NUMBER 0 1 2 REVISION DATE 7/04 3/08 6/10 Initial release Adding MAX1556A as a new version Added soldering temperature, added TOC for external feedback switching waveforms, and added paragraph discussing noise coupling when using external feedback resistors DESCRIPTION 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 (c) 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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