General Description
The MAX1644 constant-off-time, PWM step-down DCDC converter is ideal for use in applications such as PC
cards, CPU daughter cards, and desktop computer
bus-termination boards. The device features internal
synchronous rectification for high efficiency and
reduced component count. It requires no external
Schottky diode. The internal 0.10Ω PMOS power switch
and 0.10Ω NMOS synchronous-rectifier switch easily
deliver continuous load currents up to 2A. The
MAX1644 produces a preset +3.3V or +2.5V output
voltage or an adjustable output from +1.1V to V
IN
. It
achieves efficiencies as high as 95%.
The MAX1644 uses a unique current-mode, constantoff-time, PWM control scheme, which includes an Idle
Mode™ to maintain high efficiency during light-load
operation. The programmable constant-off-time architecture sets switching frequencies up to 350kHz, allowing the user to optimize performance trade-offs
between efficiency, output switching noise, component
size, and cost. The device also features an adjustable
soft-start to limit surge currents during start-up, a 100%
duty cycle mode for low-dropout operation, and a lowpower shutdown mode that disconnects the input from
the output and reduces supply current below 1µA. The
MAX1644 is available in a 16-pin SSOP package.
Applications
+5V to +3.3V/+2.5V Conversion
CPU I/O Supply
+3.3V PC Card and CardBus Applications
Notebook and Subnotebook Computers
Desktop Bus-Termination Boards
CPU Daughter Card Supply
Features
♦ ±1% Output Accuracy
♦ 95% Efficiency
♦ Internal PMOS and NMOS Switches
70mΩ On-Resistance at V
IN
= +4.5V
100mΩ On-Resistance at VIN= +3V
♦ Output Voltage
+3.3V or +2.5V Pin-Selectable
+1.1V to V
IN
Adjustable
♦ +3V to +5.5V Input Voltage Range
♦ 360µA (max) Operating Supply Current
♦ < 1µA Shutdown Supply Current
♦ Programmable Constant-Off-Time Operation
♦ 350kHz (max) Switching Frequency
♦ Idle Mode Operation at Light Loads
♦ Thermal Shutdown
♦ Adjustable Soft-Start Inrush Current Limiting
♦ 100% Duty Cycle During Low-Dropout Operation
♦ Output Short-Circuit Protection
♦ 16-Pin SSOP Package
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
SHDN LX
PGND
LX
PGND
V
CC
FBSEL
REF
GND
TOP VIEW
MAX1644
SSOP
IN
LX
COMP
IN
SS
TOFF
FB
19-1457; Rev 2; 3/02
PART
MAX1644EAE -40°C to +85°C
TEMP RANGE PIN-PACKAGE
16 SSOP
Pin Configuration
Ordering Information
Idle Mode is a trademark of Maxim Integrated Products, Inc.
Typical Operating Circuit
TOFF
COMP
V
CC
FBSEL
SHDN
IN
PGND
GND
REF
SS
LX
FB
MAX1644
R
TOFF
OUTPUT
+1.1V TO
V
IN
INPUT
+3V TO
+5.5V
________________________________________________________________ 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.
EVALUATION KIT
AVAILABLE
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= VCC= +3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
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.
VCC, IN to GND ........................................................-0.3V to +6V
IN to V
CC
.............................................................................±0.3V
GND to PGND.....................................................................±0.3V
All Other Pins to GND.................................-0.3V to (V
CC
+ 0.3V)
LX Current (Note 1)...........................................................±3.75A
REF Short Circuit to GND Duration ............................Continuous
ESD Protection .....................................................................±2kV
Continuous Power Dissipation (T
A
= +70°C)
SSOP (derate 16.7mW/°C above +70°C;
part mounted on 1 in.
2
of 1oz. copper)............................1.2W
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10sec) ............................ +300°C
Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias these diodes should take care not to exceed
the IC’s package power dissipation limits.
Hysteresis = 15°C
SHDN = GND
SHDN = GND
I
LOAD
= 0 to
2A,
VFB= V
OUT
VFB= 1.2V
(Note 2)
VIN= VCC= 3V, I
LOAD
= 1A, FBSEL = V
CC
ILX= 0.5A
FBSEL = REF, VCC, or unconnected
VIN= VCC= 3V to 5.5V,
I
LOAD
= 0, FBSEL = GND or REF
FBSEL = GND
I
REF
= -1µA to +10µA
CONDITIONS
°C150T
SHDN
Thermal Shutdown
Threshold
µA15I
IN
PMOS Switch Off-Leakage
Current
µA<1 3I
CC(SHDN)
Shutdown Supply Current
µA240 360IIN+ I
CC
No-Load Supply Current
kHz350fSwitching Frequency
A0.25 0.45 0.65I
IM
Idle Mode Current
Threshold
A2.5 2.9 3.3I
LIMIT
Current-Limit Threshold
mΩ
100 200
R
ON, P
PMOS Switch
On-Resistance
70 150
mV0.5 1∆V
REF
Reference Load Regulation
2.500 2.525 2.550 V
3.300 3.333 3.366
V3.0 5.5VIN, V
CC
Input Voltage
V1.089 1.100 1.111V
REF
Reference Voltage
mV200V
DO
Dropout Voltage
%
0.4
1.089 1.100 1.111
V
OUT
Preset Output Voltage
Adjustable Output Voltage
Range
VV
REF
V
IN
0.2
DC Load Regulation Error
UNITSMIN TYP MAXSYMBOLPARAMETER
VIN= VCC= 4V to 5.5V,
FBSEL = unconnected
VIN= VCC= 3V to 5.5V,
FBSEL = V
CC
VIN= VCC= 3V to 5.5V,
FBSEL = REF
VIN= 4.5V
VIN= 3V
ILX= 0.5A mΩ
100 200
R
ON, N
NMOS Switch
On-Resistance
70 150VIN= 4.5V
VIN= 3V
FBSEL = GND
FBSEL = REF, VCC, or unconnected
1
AC Load Regulation Error %
2
A2.5RMS LX Output Current
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= VCC= +3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
ELECTRICAL CHARACTERISTICS
(VIN= VCC= +3.3V, FBSEL = GND, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 3)
Note 2: Recommended operating frequency, not production tested.
Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
R
TOFF
= 150kΩ
VFB= 1.2V
VFB= 1.2V
ILX= 0.5A
VIN= 3.0V to 5.5V, I
LOAD
= 0,
FBSEL = GND or REF
ILX= 0.5A
CONDITIONS
µs1.03 1.63t
OFF
Off-Time Default Period
nA0 250I
FB
FB Input Bias Current
µA360IIN+ I
CC
No-Load Supply Current
A0.2 0.7I
IM
Idle Mode Current
Threshold
A2.3 3.5I
LIMIT
Current-Limit Threshold
mΩ
200
R
ON, P
PMOS Switch
On-Resistance
mΩ
150
200
R
ON, N
2.48 2.57
3.276 3.390
V3.0 5.5V
IN
Input Voltage
V1.08 1.12V
REF
Reference Voltage
VV
REF
V
IN
Adjustable Output Voltage
1.08 1.12
UNITSMIN TYP MAXSYMBOLPARAMETER
VIN= VCC= 4V to 5.5V,
FBSEL = unconnected
VIN= 3V to 5.5V, FBSEL = V
CC
VIN= 3V to 5.5V, FBSEL = REF
VIN= 4.5V
VIN= 3V
NMOS Switch
On-Resistance
150VIN= 4.5V
VIN= 3V
V
I
LOAD
= 0 to 2A,
V
FB
= V
OUT
V
OUT
Preset Output Voltage
Maximum Output RMS
Current
A
RMS
SS Sink Current I
SS
100 µA
SHDN Input Current
I
SHDN
-0.5 0.5 µA
SS Source Current I
SS
3.5 5 6.5 µA
SHDN Input Low Threshold
V
IL
0.8 V
SHDN Input High Threshold
V
IH
2.0 V
FBSEL Input Current -5 +5 µA
0.9 1.3
5.8
VSS= 1V
V
SHDN
= 0 to V
CC
FBSEL = REF
PARAMETER SYMBOL MIN TYP MAX UNITS
Undervoltage Lockout
Threshold
V
UVLO
2.5 2.6 2.7 V
FB Input Bias Current I
FB
0 80 200 nA
t
OFF
1.13 1.33 1.53
0.20 0.33
Off-Time Start-Up Period t
OFF
4 · t
OFF
µs
On-Time Period t
ON
0.4 µs
CONDITIONS
VFB= 1.2V
R
TOFF
= 150kΩ
R
TOFF
= 30.1kΩ
VINfalling, hysteresis = 40mV
FB = GND
Off-Time Default Period
4.3 5.6
µs
R
TOFF
= 499kΩ
FBSEL Logic Thresholds
0.2
V
FBSEL = GND
0.7 · V
CC
0.7 · V
CC
- 0.2 + 0.2
FBSEL = unconnected
VCC- 0.2FBSEL = V
CC
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
4 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)
110
0
10
30
20
40
50
70
60
80
90
100
0.001 0.01 0.1
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-01
OUTPUT CURRENT (A)
EFFICIENCY (%)
VIN = 5V, V
OUT
= 3.3V,
L = 6.0µH, R
TOFF
= 120kΩ
100
110
0
10
30
20
40
50
70
60
80
90
0.001 0.01 0.1
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-02
OUTPUT CURRENT (A)
EFFICIENCY (%)
VIN = 3.3V, V
OUT
= 1.5V,
L = 4.7µH, R
TOFF
= 200kΩ
VIN = 5V, V
OUT
= 1.5V,
L = 6.0µH, R
TOFF
= 270kΩ
110
-1.0
-0.9
-0.7
-0.8
-0.6
-0.5
-0.3
-0.4
-0.2
-0.1
0
0.0001 0.001 0.01 0.1
DC LOAD-REGULATION ERROR
vs. OUTPUT CURRENT
MAX1644-03
OUTPUT CURRENT (A)
DC LOAD-REGULATION ERROR (%)
A
B
E
C
D
A: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7µH,
R
TOFF
= 200kΩ, FBSEL = GND
B: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7µH,
R
TOFF
= 200kΩ, FBSEL = REF
C: V
IN
= 5V, V
OUT
= 3.3V, L = 6.0µH,
R
TOFF
= 120kΩ, FBSEL = OPEN
D: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0µH,
R
TOFF
= 270kΩ, FBSEL = GND
E: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0µH,
R
TOFF
= 270kΩ, FBSEL = REF
0
100
50
250
200
150
300
350
0 1.00.5 1.5 2.0
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1644-04
OUTPUT CURRENT (A)
SWITCHING FREQUENCY (kHz)
VIN = 3.3V,
V
OUT
= 1.5V,
L = 4.7µH,
R
TOFF
= 200kΩ
VIN = 5V,
V
OUT
= 1.5V,
L = 6.0µH,
R
TOFF
= 270kΩ
VIN = 5V,
V
OUT
= 3.3V,
L = 6.0µH,
R
TOFF
= 120kΩ
0
150
100
50
200
250
300
350
400
450
500
021 3456
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1644-07
SUPPLY VOLTAGE
SUPPLY CURRENT I
CC
(µA)
0
0.03
0.02
0.01
0.04
0.05
0.06
0.07
0.08
0.09
0.10
SHUTDOWN SUPPLY CURRENT I
IN
+ I
CC
(µA)
I
OUT
= 0
UNDERVOLTAGE
LOCKOUT
SHDN = VIN = V
CC
SHDN = GND
0
1.0
0.5
2.5
2.0
1.5
4.0
3.5
3.0
4.5
0 200100 300 400 500 600
OFF-TIME vs. R
TOFF
MAX1644-06
R
TOFF
(kΩ)
t
OFF
(µs)
V
SHDN
5V/div
I
IN
1A/div
0
0
V
OUT
2V/div
V
SS
1V/div
0
0
2ms/div
START-UP AND
SHUTDOWN TRANSIENTS
MAX1644-09
VIN = 5.0V, V
OUT
= 3.3V, I
OUT
= 2A
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)
NAME FUNCTION
1
SHDN
Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal
MOSFETs. Drive high or connect to V
CC
for normal operation.
PIN
Pin Description
V
IN
V
OUT
20mV/div
4V
3V
20µs/div
LINE-TRANSIENT RESPONSE
MAX1644-10
V
OUT
= 1.5V, I
OUT
= 2A
I
L
V
OUT
50mV/div
2A
0
20µs/div
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)
MAX1644-11
VIN = 3.3V, V
OUT
= 1.5V
2, 4 IN Supply Voltage Input for the internal PMOS power switch
3, 14, 16 LX
Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect
the inductor from this node to output filter capacitor and load.
5 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during start-up.
6 COMP
Integrator Compensation. Connect a capacitor from COMP to VCCfor integrator compensation. See the
Integrator Amplifier section.
7 TOFF
Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a
resistor from TOFF to GND to adjust the PMOS switch off-time.
8 FB
Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to
output for fixed-voltage operation or to a resistor-divider for adjustable operating modes.
9 GND Analog Ground
10 REF Reference Output. Bypass REF to GND with a 1µF capacitor.
11 FBSEL
Feedback Select Input. Selects AC load-regulation error and output voltage. See Table 2 for programming instructions.
12 V
CC
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCCwith a 10Ω and 2.2µF lowpass filter. See Figure 1.
13, 15 PGND Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
6 _______________________________________________________________________________________
_______________Detailed Description
The MAX1644 synchronous, current-mode, constant-offtime, PWM DC-DC converter steps down input voltages
of +3V to +5.5V to a preset output voltage of either +3.3V
or +2.5V, or to an adjustable output voltage from +1.1V
to VIN. The device delivers up to 2A of continuous load
current. Internal switches composed of a 0.1Ω PMOS
power switch and a 0.1Ω NMOS synchronous-rectifier
switch improve efficiency, reduce component count, and
eliminate the need for an external Schottky diode.
The MAX1644 optimizes performance by operating in
constant-off-time mode under heavy loads and in
Maxim’s proprietary Idle Mode under light loads. A single resistor-programmable constant-off-time control
sets switching frequencies up to 350kHz, allowing the
user to optimize performance trade-offs in efficiency,
switching noise, component size, and cost. Under lowdropout conditions, the device operates in a 100%
duty-cycle mode, where the PMOS switch remains permanently on. Idle Mode enhances light-load efficiency
by skipping cycles, thus reducing transition and gatecharge losses.
When power is drawn from a regulated supply, constantoff-time PWM architecture essentially provides constantfrequency operation. This architecture has the inherent
advantage of quick response to line and load transients.
The MAX1644’s current-mode, constant-off-time PWM
architecture regulates the output voltage by changing
the PMOS switch on-time relative to the constant offtime. Increasing the on-time increases the peak inductor current and the amount of energy transferred to the
load per pulse.
Modes of Operation
The current through the PMOS switch determines the
mode of operation: constant-off-time mode (for load
currents greater than 0.2A) or Idle Mode (for load currents less than 0.2A). Current sense is achieved
through a proprietary architecture that eliminates current-sensing I2R losses.
Constant-Off-Time Mode
Constant-off-time operation occurs when the current
through the PMOS switch is greater than the Idle Mode
threshold current (0.4A, which corresponds to a load
current of 0.2A). In this mode, the regulation comparator turns the PMOS switch on at the end of each offtime, keeping the device in continuous-conduction
mode. The PMOS switch remains on until the output is
in regulation or the current limit is reached. When the
PMOS switch turns off, it remains off for the pro-
grammed off-time (t
OFF
). If the output falls dramatically
out of regulation—approximately VFB/ 4—the PMOS
switch remains off for approximately four times t
OFF
.
The NMOS synchronous rectifier turns on shortly after
the PMOS switch turns off, and it remains on until shortly before the PMOS switch turns back on.
Idle Mode
Under light loads, the device improves efficiency by
switching to a pulse-skipping Idle Mode. Idle Mode
operation occurs when the current through the PMOS
switch is less than the Idle Mode threshold current. Idle
Mode forces the PMOS to remain on until the current
through the switch reaches 0.4A, thus minimizing the
unnecessary switching that degrades efficiency under
light loads. In Idle Mode, the device operates in discontinuous conduction. Current-sense circuitry monitors the
current through the NMOS synchronous switch, turning it
off before the current reverses. This prevents current
from being pulled from the output filter through the
inductor and NMOS switch to ground. As the device
switches between operating modes, no major shift in circuit behavior occurs.
100% Duty-Cycle Operation
When the input voltage drops near the output voltage,
the duty cycle increases until the PMOS MOSFET is on
continuously. The dropout voltage in 100% duty cycle
is the output current multiplied by the on-resistance of
the internal PMOS switch and parasitic resistance in the
inductor. The PMOS switch remains on continuously as
long as the current limit is not reached.
Shutdown
Drive SHDN to a logic-level low to place the MAX1644
in low-power shutdown mode and reduce supply current to less than 1µA. In shutdown, all circuitry and
internal MOSFETs turn off, and the LX node becomes
high impedance. Drive SHDN to a logic-level high or
connect to VCCfor normal operation.
Summing Comparator
Three signals are added together at the input of the
summing comparator (Figure 1): an output voltage error
signal relative to the reference voltage, an integrated
output voltage error correction signal, and the sensed
PMOS switch current. The integrated error signal is provided by a transconductance amplifier with an external
capacitor at COMP. This integrator provides high DC
accuracy without the need for a high-gain amplifier.
Connecting a capacitor at COMP modifies the overall
loop response (see the Integrator Amplifier section).
Synchronous Rectification
In a step-down regulator without synchronous rectification, an external Schottky diode provides a path for current to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS synchronous switch reduces conduction losses and
improves efficiency.
The NMOS synchronous-rectifier switch turns on following a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In
constant-off-time mode, the synchronous-rectifier
switch turns off just prior to the PMOS power switch
turning on. While both switches are off, inductor current
flows through the internal body diode of the NMOS
switch. The internal body diode’s forward voltage is relatively high.
Thermal Resistance
Junction-to-ambient thermal resistance, θJA, is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1644 evaluation kit
has 0.5 in.2of copper area and a thermal resistance of
60°C/W with no airflow. Airflow over the IC significantly
reduces the junction-to-ambient thermal resistance. For
heatsinking purposes, evenly distribute the copper area
connected at the IC among the high-current pins.
Power Dissipation
Power dissipation in the MAX1644 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to supply current in the control
section and average current used to charge and discharge the gate capacitance of the internal switches
are less than 30mW at 300kHz. This number is reduced
when the switching frequency decreases as the part
enters Idle Mode. Combined conduction losses in the
two power switches are approximated by:
PD= I
OUT
2
· R
ON
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
θJA= (T
J,MAX
- T
A,MAX
) / P
D
where: θJA= junction-to-ambient thermal resistance
T
J,MAX
= maximum junction temperature
T
A,MAX
= maximum ambient temperature
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 7
MAX1644
V
CC
470pF
2.2µF
1µF
10µF
10Ω
FBSEL
0.01µF
FEEDBACK
SELECTION
CURRENT
SENSE
PWM LOGIC
AND
DRIVERS
SS
IN
FB
V
IN
3.0V TO 5.5V
LX
PGNDTOFF
R
TOFF
GND
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
REF
REF
SUMMING
COMPARATOR
REF
REF
COMP
SKIP
SHDN
TIMER
V
IN
CURRENT
SENSE
G
m
C
OUT
V
OUT
Figure 1. Functional Diagram
MAX1644
__________________Design Procedure
For typical applications, use the recommended component values in Table 1. For other applications, take the
following steps:
1) Select the desired PWM-mode switching frequency;
300kHz is a good starting point.
2) Select the constant-off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R
TOFF
as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
The output of the MAX1644 is selectable between one
of two preset output voltages: (2.5V or 3.3V) with a 2%
AC load-regulation error, or an adjustable output voltage from the reference voltage (nominally 1.1V) up to
VINwith a 1% or 2% AC load-regulation error. For a
preset output voltage, connect FB to the output voltage,
and connect FBSEL to VCC(2.5V output voltage) or
leave unconnected (3.3V output voltage). Internal resistor-dividers divide down the output voltage, regulating
the divided voltage to the internal reference voltage.
For output voltages other than 2.5V or 3.3V, or for
tighter AC load regulation, connect FBSEL to GND (1%
regulation) or to REF (2% regulation), and connect FB
to a resistor divider between the output voltage and
ground (Figure 2). Regulation is maintained for
adjustable output voltages when V
FB
equals V
REF
. Use
50kΩ for R1. R2 is given by the equation:
where V
REF
is typically 1.1V.
Programming the Switching Frequency
and Off-Time
The MAX1644 features a programmable PWM mode
switching frequency, which is set by the input and output voltage and the value of R
TOFF
, connected from
TOFF to GND. R
TOFF
sets the PMOS power switch offtime in PWM mode. Use the following equation to select
the off-time according to your desired switching frequency in PWM mode (I
OUT
> 0.2A):
where: t
OFF
= the programmed off-time
VIN= the input voltage
V
OUT
= the output voltage
V
NMOS
= the voltage drop across the internal
PMOS power switch
V
PMOS
= the voltage drop across the internal
NMOS synchronous-rectifier switch
t
VV V
fVV V
OFF
IN OUT PMOS
PWM IN PMOS NMOS
–
=
−
()
−+
()
R2 R1
V
V
1
OUT
REF
=−
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
8 _______________________________________________________________________________________
V
OUT
(V)
R
TOFF
(kΩ)
6.0 120
L
(µH)
5 3.3
6.8
V
IN
(V)
180
6.8 2405 1.8
3.3 823.3 2.5
4.7 1803.3 1.8
4.7 2003.3 1.5
5 2.5
Table 2. Output Voltage and AC LoadRegulation Selection
PIN
2.5 2V
CC
Output
Voltage
3.3 2
Adjustable 2REF
Resistor
Divider
Adjustable 1GND
Resistor
Divider
Unconnected
Output
Voltage
FB
AC LOAD-
REGULATION
ERROR (%)
OUTPUT
VOLTAGE
(V)
FBSEL
Figure 2. Adjustable Output Voltage
Table 1. Recommended Component
Values (I
OUT
= 2A, f
PWM
= 300kHz)
6.0 2705 1.5
LX
MAX1644
FB
R1 = 50kΩ
R2 = R1(V
V
REF
= 1.1V
OUT
/ V
- 1)
REF
V
R2
R1
OUT
f
PWM
= switching frequency in PWM mode
(I
OUT
> 0.2A)
Select R
TOFF
according to the formula:
R
TOFF
= (t
OFF
- 0.07µs) (150kΩ / 1.26µs)
Recommended values for R
TOFF
range from 39kΩ to
470kΩ for off-times of 0.4µs to 4µs.
Inductor Selection
Three key inductor parameters must be specified:
inductor value (L), peak current (I
PEAK
), and DC resistance (RDC). The following equation includes a constant, denoted as LIR, which is the ratio of peakto-peak inductor AC current (ripple current) to maximum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and ripple. A good compromise between size and losses is
found at approximately a 25% ripple-current to loadcurrent ratio (LIR = 0.25), which corresponds to a peak
inductor current 1.125 times higher than the DC load
current:
where: I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
The peak inductor current at full load is 1.125 · I
OUT
if
the above equation is used; otherwise, the peak current
is calculated by:
Choose an inductor with a saturation current at least as
high as the peak inductor current. To minimize loss,
choose an inductor with a low DC resistance.
Capacitor Selection
The input filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and lowESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:
The output filter capacitor affects the output voltage ripple, output load-transient response, and feedback loop
stability. For stable operation, the MAX1644 requires a
minimum output ripple voltage of V
RIPPLE
≥ 2% · V
OUT
(with 2% load regulation setting).
The minimum ESR of the output capacitor should be:
Stable operation requires the correct output filter
capacitor. When choosing the output capacitor, ensure
that:
C
OUT
≥ (t
OFF
/ V
OUT
) ✕(64µFV / µs)
With an AC load regulation setting of 1%, the C
OUT
requirement doubles, and the minimum ESR of the output capacitor is halved.
Integrator Amplifier
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C
COMP
, from COMP
to VCCcompensates the transconductance amplifier.
For stability, choose:
C
COMP
≥ 470pF
A large capacitor value maintains a constant average
output voltage but slows the loop response to changes
in output voltage. A small capacitor value speeds up
the loop response to changes in output voltage but
decreases stability. Choose the capacitor values that
result in optimal performance.
Setting the AC Loop Gain
The MAX1644 allows selection of a 1% or 2% AC loadregulation error when the adjustable output voltage
mode is selected (Table 2). A 2% setting is automatically selected in preset output voltage mode (FBSEL
connected to VCCor unconnected). A 2% load-regulation error setting reduces output filter capacitor requirements, allowing the use of smaller and less expensive
capacitors. Selecting a 1% load-regulation error
reduces transient load errors, but requires larger capacitors.
ESR
L
t
OFF
% >×1
II
VV V
V
RIPPLE LOAD
OUT IN OUT
IN
=−
()
II
Vt
L
PEAK OUT
OUT OFF
=+
×
×2
L
Vt
I LIR
OUT OFF
OUT
=
×
×
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 9
MAX1644
2A Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
10 ______________________________________________________________________________________
Soft-Start
Soft-start allows a gradual increase of the internal current limit to reduce input surge currents at start-up and
at exit from shutdown. A charging capacitor, CSS,
placed from SS to GND sets the rate at which the internal current limit is changed. Upon power-up, when the
device comes out of undervoltage lockout (2.6V typ) or
after the SHDN pin is pulled high, a 5µA constant-current source charges the soft-start capacitor and the
voltage on SS increases. When the voltage on SS is
less than approximately 0.7V, the current limit is set to
zero. As the voltage increases from 0.7V to approximately 1.8V, the current limit is adjusted from 0 to 2.9A.
The voltage across the soft-start capacitor changes
with time according to the equation:
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
I
LIMIT
= (VSS- 0.7V) · 2.7A/V, for VSS> 0.7V
The constant-current source stops charging once the
voltage across the soft-start capacitor reaches 1.8V
(Figure 3).
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1644’s
intended output power level, high efficiency, and low
noise. Good layout includes the use of a ground plane,
appropriate component placement, and correct routing
of traces using appropriate trace widths. The following
points are in order of decreasing importance:
1) Minimize switched-current and high-current ground
loops. Connect the input capacitor’s ground, the output capacitor’s ground, and PGND together.
2) Connect the input filter capacitor less than 5mm
away from IN. The connecting copper trace carries
large currents and must be at least 2mm wide,
preferably 5mm.
3) Place the LX node components as close together
and as near to the device as possible. This reduces
resistive and switching losses as well as noise.
4) A ground plane is essential for optimum performance. In most applications, the circuit is located on
a multilayer board, and full use of the four or more
layers is recommended. Use the top and bottom layers for interconnections and the inner layers for an
uninterrupted ground plane.
V
At
C
SS
SS
=×5µ
___________________Chip Information
TRANSISTOR COUNT: 1758
0.7V
1.8V
2.9A
t
SHDN
0
0
0
V
SS
(V)
I
LIMIT
(A)
Figure 3. Soft-Start Current Limit over Time
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 11
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
SSOP.EPS
MAX1644
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.)
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