19-1724; Rev 0; 5/00
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
Features
♦ On-Chip 10A Power MOSFET
♦ 5V, 4A Output from a 3.3V Input
♦ Fixed 3.3V or 5V Output Voltage or
Adjustable (2.5V to 5.5V)
♦ Input Voltage Range Down to 0.7V
♦ Low Power Consumption
1mW Quiescent Power
1µA Current in Shutdown Mode
♦ Low-Noise, Constant Frequency Operation
(600kHz)
♦ Synchronizable Switching Frequency
(350kHz to 1000kHz)
Ordering Information
________________________Applications
Routers, Servers, Workstations, Card Racks
PART
MAX1709ESE
TEMP. RANGE
PIN-PACKAGE
-40°C to +85°C
16 Narrow SO
Local 2.5V to 3.3V or 5V Conversion
Local 3.3V to 5V Conversion
3.6V or 5V RF PAs in Communications Handsets
Typical Operating Circuit
INPUT
1V TO 5V
TOP VIEW
1µH
OFF ON
SYNC
OR
INTERNAL
Pin Configuration
ONA
LX
MAX1709
CLK
GND
SS/LIM
REF
OUT
OUTPUT
3.3V, 5V,
OR ADJ
UP TO 4A
ONA 1
16 ONB
LX 2
15 CLK
LX 3
LX 4
14 3.3/5
MAX1709
13 PGND
GND 5
12 PGND
SS/LIM 6
11 PGND
REF 7
10 FB
GND 8
9
OUT
NARROW SO
________________________________________________________________ Maxim Integrated Products
1
For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX1709
General Description
The MAX1709 sets a new standard of space savings for
high-power, step-up DC-DC conversion. It delivers up
to 20W at a fixed (3.3V or 5V) or adjustable (2.5V to
5.5V) output, using an on-chip power MOSFET from a
+0.7V to +5V supply.
Fixed-frequency PWM operation ensures that the
switching noise spectrum is constrained to the 600kHz
fundamental and its harmonics, allowing easy postfiltering for noise reduction. External clock synchronization
capability allows for even tighter noise spectrum control. Quiescent power consumption is less than 1mW to
extend operating time in battery-powered systems.
Two control inputs (ONA, ONB) allow simple push-on,
push-off control through a single momentary pushbutton switch, as well as conventional on/off logic control.
The MAX1709 also features programmable soft-start
and current limit for design flexibility and optimum performance with batteries. The maximum RMS switch current rating is 10A. For a device with a lower current
rating, smaller size, and lower cost, refer to the
MAX1708 data sheet.
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
ONA, ONB, OUT, SS/LIM, 3.3/5 to GND ...............-0.3V to +6.0V
LX to PGND ...........................................................-0.3V to +6.0V
FB, CLK, REF to GND.............................. -0.3V to (VOUT + 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (TA = +70°C)
16-Pin Narrow SO (derate 16.5mW/°C above +70°C) .....1.3W
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature ........................................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°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.
ELECTRICAL CHARACTERISTICS
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
UNITS
3.3/5 = GND, ISW = 1A
3.26
3.34
3.42
3.3/5 = OUT, ISW = 1A
4.92
5.05
5.17
-0.25
-0.45
1.240
1.265
V
1
200
nA
5.5
V
Output Voltage
VFB < 0.1V (Note 1)
Load Regulation
Measured between 1A < ISW < 3A (Note 2)
FB Regulation Voltage
ISW = 1A
FB Input Current
VFB = +1.5V
Output Voltage Adjust Range
1.215
2.5
V
%/A
Output Undervoltage Lockout
(Note 3)
2.0
2.3
V
Frequency in Startup Mode
VOUT = 1.5V
40
400
kHz
Minimum Startup Voltage
IOUT < 1mA (Note 1), TA = +25°C (Note 4)
1.1
V
Minimum Operating Voltage
(Note 5)
Soft-Start Pin Current
VSS/LIM = 1V
4
5.0
µA
OUT Supply Current
VFB = 1.5V (Note 6)
200
440
µA
OUT Leakage Current In
Shutdown
LX Leakage Current
N-Channel Switch
On-Resistance
V ONB = 3.6V
0.1
5
µA
VLX = V ONB = VOUT = +5.5V
0.1
40
µA
22
40
mΩ
N-Channel Current Limit
SS/LIM = open
SS/LIM = 150k to GND
0.9
0.7
3.2
Reference Voltage
Reference Load Regulation
Reference Supply Rejection
V
7.5
9
12
3.5
5
6.5
RMS Switch Current
10
IREF = 0
-1µA ″ IREF ″ 50µA
+2.5V ″ VOUT ″ +5.5V
1.245
A
ARMS
1.260
1.275
V
4
10
mV
5
mV
0.2
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V
0.2 x
VOUT
CLK, 2.7V < VOUT < 5.5V
0.2 x
VOUT
Input Low Level (Note 7)
2
MAX
_______________________________________________________________________________________
V
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
Input High Level
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5
CLK, 2.7 V< VOUT < 5.5V
Logic Input Current
ONA, ONB, CLK, 3.3/5
MIN
TYP
MAX
0.8 x VOUT
0.8 x VOUT
UNITS
V
1
µA
Internal Oscillator Frequency
520
600
680
kHz
Maximum Duty Cycle
External Clock Frequency
Range
CLK Pulse Width
82
90
94
%
1000
kHz
(Note 8)
CLK Rise/Fall Time
(Note 8)
350
100
ns
50
ns
MAX
UNITS
ELECTRICAL CHARACTERISTICS
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER
Output Voltage
CONDITIONS
MIN
3.3/5 = GND, ISW = 1A
3.24
3.45
(Note 1)
3.3/5 = OUT, ISW = 1A
4.9
5.2
1.21
1.27
V
200
nA
-0.45
%/A
5.2
µA
5
µA
400
µA
40
mΩ
FB Regulation Voltage
ISW = 1A
FB Input Current
VFB = +1.5V
Load Regulation
Measured between 1A < ISW < 5A (Note 2)
Soft-Start Pin Current
SS/LIM = 1V
OUT Leakage Current in
Shutdown
V ONB = 3.6V
OUT Supply Current
VFB = 1.5V (Note 6)
3.2
N-Channel Switch
On-Resistance
N-Channel Current Limit
Reference Voltage
TYP
VFB < 0.1V, VIN = +2.4V
SS/LIM = unconnected
7.5
15
SS/LIM = 150kΩ to GND
3.5
7
IREF = 0
1.24
1.28
V
V
V
_______________________________________________________________________________________
3
MAX1709
ELECTRICAL CHARACTERISTICS (continued)
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V
0.2 ×
VOUT
V
CLK, 2.7V < VOUT < 5.5V
0.2 ×
VOUT
Input Low Level (Note 7)
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V
0.8 ×
VOUT
CLK, VOUT = 5.5V
0.8 ×
VOUT
Input High Level
V
1
µA
Internal Oscillator Frequency
500
700
kHz
Maximum Duty Cycle
80
95
%
External Clock Frequency
Range
350
1000
kHz
Logic Input Current
ONA, ONB, CLK, 3.3/5
CLK/SEL Pulse Width
(Note 8)
CLK/SEL Rise/Fall Time
(Note 8)
100
ns
50
ns
Note 1: Output voltage is specified at 1A switch current ISW, which is equivalent to approximately 1A ✕ (VIN / VOUT) of load current.
Note 2: Load regulation is measured by forcing specified switch current and straight-line calculation of change in output voltage in
external feedback mode. Note that the equivalent load current is approximately ISW ✕ (VIN / VOUT).
Note 3: Until undervoltage lockout is reached, the device remains in startup mode. Do not apply full load until this voltage is
reached.
Note 4: Startup is tested with Figure 1’s circuit. Output current is measured when both the input and output voltages are applied.
Note 5: Minimum operating voltage. The MAX1709 is bootstrapped and will operate down to a 0.7V input once started.
Note 6: Supply current is measured from the OUT pin to the output voltage (+3.3V). This correlates directly with actual input supply
current but is reduced in value according to the step-up ratio and efficiency.
Note 7: ONA and ONB inputs have approximately 0.15V hysteresis.
Note 8: Guaranteed by design, not production tested.
Note 9: Specifications to -40°C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
EFFICIENCY (%)
VIN = 1.2V
60
50
40
70
50
40
30
20
20
10
10
0
VIN = 2.5V
60
30
85.5
0.01
0.1
1
10
0.01
0.1
1
LOAD REGULATION
(VIN = 3.3V, VOUT = 5V)
LINE REGULATION
(VOUT = 5V, IOUT = 1A)
0.5
0.6
0.7
0.8
0.9
1.0
NO-LOAD INPUT CURRENT
vs. INPUT VOLTAGE
1000
MAX1709 TOC05
0.6
0.4
INPUT VOLTAGE
INCREASING
INPUT CURRENT (mA)
VOUT REGULATION (%)
0.4
OPERATING FREQUENCY (MHz)
0.8
MAX1709 TOC04
0
84.0
10
OUTPUT CURRENT (A)
1
84.5
83.0
0.001
OUTPUT CURRENT (A)
2
85.0
83.5
0
0.001
VOUT REGULATION (%)
VIN = 3.3V
80
86.0
0.2
0
-0.2
MAX1709 TOC06
70
90
EFFICIENCY (%)
VIN = 2.5V
80
EFFICIENCY (%)
100
EFFICIENCY vs. SWITCHING FREQUENCY
(VIN = 3.3V, VOUT = 5V, IOUT = 2A)
MAX1709 TOC02
90
MAX1709 TOC01
100
EFFICIENCY vs.
OUTPUT CURRENT (VOUT = 5V)
MAX1709 TOC03
EFFICIENCY vs.
OUTPUT CURRENT (VOUT = 3.3V)
100
INPUT VOLTAGE
DECREASING
10
VOUT = 5V
-0.4
-1
1
-0.6
2
3
4
MAX1709 TOC07
2.0
1.8
VOUT = 3.3V
1.6
1.4
TA = -40°C
1.0
TA = +25°C
0.01
0.5
VOUT = 5V
0.1
1
10
2.0
1.5
2.5
3.0
3.5
25
20
-0.5
-1.0
15
10
5
0
VOUT = 3.3V
-5
-10
-2.0
LOAD CURRENT (A)
1.0
NOISE vs. FREQUENCY
0
-1.5
TA = +85°C
0.5
30
NOISE (mVRMS)
2.2
0
4.0
INPUT VOLTAGE (V)
1.0
FREQUENCY CHANGE (%)
STARTUP VOLTAGE (V)
3.5
SWITCHING FREQUENCY
vs. TEMPERATURE
2.4
0.001
3.0
STARTUP VOLTAGE
vs. LOAD CURRENT
2.6
0.8
2.5
INPUT VOLTAGE (V)
2.8
0.6
2.0
5
OUTPUT CURRENT (A)
MAX1709 TOC09
1
MAX1709 TOC08
0
1.2
VOUT = 3.3V
0.1
-0.8
-2
-40
-15
10
35
TEMPERATURE (°C)
60
85
0.1
1
10
FREQUENCY (MHz)
_______________________________________________________________________________________
5
MAX1709
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
SWITCH CURRENT LIMIT
vs. TEMPERATURE
SWITCH CURRENT LIMIT
vs. SS/LIM RESISTANCE
11.0
CURRENT LIMIT (A)
8
7
6
5
MAX1709-13a
MAX1709-12
9
HEAVY SWITCHING WAVEFORM
11.5
MAX1709 TOC10
10
CURRENT LIMIT (A)
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
5V
10.0
VOUT
100mV/div
5V
9.5
9.0
VOUT = 3.3V
4
4A
8.5
3
VLX
5V/div
0
10.5
IL
2A/div
2A
IOUT = 2A
8.0
2
50
100
150
200
250
-40
300
-15
10
35
60
85
1µs/div
TEMPERATURE (°C)
SS/LIM RESISTANCE (kΩ)
LINE-TRANSIENT
RESPONSE
HEAVY SWITCHING WAVEFORM
(WITH LC FILTER)
MAX1709-14
MAX1709-13b
5V
VLX
5V/div
0
3.5V
VIN
0.5V/div
3V
VOUT
100mV/div
5V
4A
IL
2A/div
2A
VOUT
50mV/div
5V
IOUT = 1A
100µs/div
1µs/div
IOUT = 2A
L = 12.5nH (COILCRAFT A04T)
C = 1µF
SHUTDOWN WITH SOFT-START
(CSS = 0.1µF)
LOAD-TRANSIENT
RESPONSE
MAX1709-16
MAX1709-15
3A
5V
IOUT
2A/div
1A
VOUT
100mV/div
5V
VONA
5V/div
0
4A
IIN
2A/div
2A
6A
0
IL
2A/div
4A
2A
VOUT
2V/div
4V
2V
1ms/div
20µs/div
6
CSS = 0.1µF
ROUT = 5Ω
VONB = VOUT
_______________________________________________________________________________________
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
SHUTDOWN WITH SOFT-START
(CSS = 0.01µF)
SHUTDOWN WITHOUT SOFT-START
MAX1709-18
MAX1709-17
5V
VONA
5V/div
0
5V
VONA
5V/div
0
6A
6A
4A
IIN
2A/div
2A
4A
IIN
2A/div
2A
0
0
VOUT
2V/div
4V
VOUT
2V/div
4V
2V
2V
1ms/div
1ms/div
CSS = 0
ROUT = 5Ω
VONB = VOUT
CSS = 0.01µF
ROUT = 5Ω
VONB = VOUT
Pin Description
PIN
NAME
1
ONA
2, 3, 4
LX
5, 8
GND
FUNCTION
On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1).
Drain of N-Channel Power Switch. Connect pins 2, 3, and 4 together. Connect external
Schottky diode from LX to OUT.
Ground
Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the
rate at which the device reaches current limit (soft-start). To reduce the current limit from the
preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During
shutdown, this pin is internally pulled to GND to discharge the soft-start capacitor.
6
SS/LIM
7
REF
1.26V Voltage Reference Output. Bypass with a 0.22µF capacitor to GND. Maximum REF load
is 50µA.
9
OUT
Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1µF to PGND
with less than 5mm trace length. Connect a 2Ω series resistor from the output filter capacitor to
OUT (Figure 1).
10
FB
11, 12, 13
PGND
14
3.3/5
Output Voltage Selection Pin. When FB is connected to GND, the regulator uses internal
feedback to set the output voltage. 3.3/5 = low sets output to 3.3V; 3.3/5 = high sets output to
5V. If an external divider is used at FB, connect 3.3/5 to ground.
15
CLK
Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Optionally, drive
with an external clock for external synchronization.
16
ONB
Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).
DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see
3.3/5 pin description). Connect a resistor-divider from the output to set the output voltage in
the +2.5V to +5.5V range. FB regulates to +1.25V (Figure 4).
Power Ground, Source of N-Channel Power MOSFET Switch
_______________________________________________________________________________________
7
MAX1709
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
KEEP TRACES
SHORT AND WIDE
1µH
L1
VIN
1V TO 5V
D1
ON-OFF
CONTROL
C1, C2
2 x 150µF
ONA
ONB
LX
CLK
LX
3.3/5
LX
PGND
VOUT
5V
C6, C7
2x
150µF
GND
GND
MAX1709
SS/LIM
R1
C3
PGND
PGND
REF
FB
GND
OUT
C4
0.22µF
R2
2Ω
C5
0.1µF
Figure 1. Standard Operating Circuit
_______________Detailed Description
The MAX1709 step-up converter offers high efficiency
and high integration for high-power applications. It
operates with an input voltage as low as 0.7V and is
suitable for single- to 3 cell battery inputs as well as
2.5V or 3.3V regulated supply inputs. The output voltage is preset to +3.3V or +5.0V or can be adjusted with
external resistors for voltages between +2.5V to +5.5V.
The MAX1709 internal N-channel MOSFET switch is
rated for 10A (RMS value) and can deliver loads to 4A,
depending on input and output voltages. For flexibility,
the current limit and soft-start rate are independently
programmable.
A 600kHz switching frequency allows for a small inductor to be used. The switching frequency is also synchronizable to an external clock ranging from 350kHz
to 1000kHz.
ONA, ONB
The logic levels at ONA and ONB turn the MAX1709 on
or off. When ONA = 1 or ONB = 0, the part is on. When
ONA = 0 and ONB = 1, the part is off (Table 1). Logic
high on control can be implemented by tying ONB high
and using ONA for shutdown. Implement inverted sin8
gle-line on/off control by grounding ONA and toggling
ONB. Implement momentary pushbutton On/Off as
described in the Applications Information section. Both
inputs have approximately 0.15V of hysteresis.
Switching Frequency
The MAX1709 switches at the fixed-frequency internal
oscillator rate (600kHz) or can be synchronized to an
external clock. Connect CLK to OUT for internal clock
operation. Apply a clock signal to CLK to synchronize
to an external clock. The frequency can be changed on
the fly. The MAX1709 will synchronize to a new external
clock rate in two cycles and will take approximately
40µs to revert to its internal clock frequency once the
external clock pulses stop and CLK is driven high.
Table 2 summarizes oscillator operation.
Operation
The MAX1709 switches at a constant frequency
(600kHz) and modulates the MOSFET switch pulse
width to control the power transferred per cycle and
regulate the voltage across the load. In low-noise applications, the fundamental and the harmonics generated
by the fixed switching frequency are easily filtered out.
Figure 2 shows the simplified functional diagram for the
MAX1709. Figure 3 shows the simplified PWM con-
_______________________________________________________________________________________
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
MAX1709
Table 1. On/Off Logic Control
Table 2. Selecting Switching Frequency
ONA
0
ONB
0
MAX1709
On
0
1
Off
1
0
On
1
1
On
CLK
0
1
External clock
(350kHz−1000kHz)
MODE
Not allowed
PWM
Synchronized PWM
UNDERVOLTAGE LOCKOUT
OUT
MAX1709
IC POWER
PWM
CONTROLLER
2.15V
EN
STARTUP
Q
OSCILLATOR
D
SEE
FIGURE 3.
ONA
ONB
REF
1.260V
EN
ON
RDY
REFERENCE
EN
CLK
FB
3.3/5
GND
DUAL MODE
FB
600kHz
OSCILLATOR
LX
OSC
N
PGND
FB
OUT
Figure 2. Simplified Functional Diagram
troller functional diagram. The MAX1709 enters synchronized current-mode PWM when a clock signal
(350kHz < f CLK < 1000kHz) is applied to CLK. For
wireless or noise-sensitive applications, this ensures
that switching harmonics are predictable and kept outside the IF frequency band(s). High-frequency operation permits low-magnitude output ripple voltage and
minimum inductor and filter capacitor size. Switching
losses will increase at the higher frequencies (see
Power Dissipation).
Setting the Output Voltage
The MAX1709 features Dual Mode™ operation. When
FB is connected to ground, the MAX1709 generates a
fixed output voltage of either +3.3V or +5V, depending
on the logic applied to the 3.3/5 input (Figure 1). The
output can be configured for other voltages, using two
external resistors as shown in Figure 4. To set the output voltage externally, choose an R3 value that is large
enough to minimize load at the output but small enough
to minimize errors due to leakage and the time constant
to FB. A value of R4 ≤ 50kΩ is required.
V
R3 = R4 OUT − 1
VFB
where VFB = 1.24V.
Soft-Start/Current-Limit Adjustment
(SS/LIM)
The soft-start pin allows the soft-start time to be adjusted by connecting a capacitor from SS/LIM to ground.
Select capacitor C3 (connected to SS/LIM pin) as:
C3 (in µF) = 3.2 ✕ tSS
where tSS is the time (in seconds) it takes the switch
current limit to reach full value.
To improve efficiency or reduce inductor size at
reduced load currents, the current limit can be reduced
from its nominal value (see Electrical Characteristics).
A resistor (R1 in Figure 1) between SS/LIM and ground
reduces the current limit as follows:
R1 = 312.5kΩ ×
I1
(R1 ≤ 312.5kΩ)
ILIM
Dual Mode is a trademark of Maxim Integrated Products.
_______________________________________________________________________________________
_______________________________________________________________________________________
9
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
Table 3. Component Selection Guide
PRODUCTION
Surface mount
INDUCTORS
CAPACITORS
Panasonic EEFUE0J151R
Motorola MBRD1035CTL
Coiltronics UP2B-1R0
Sanyo 6TPC100M
STM-Microelectronics
STPS8L30B
Table 4. Component Suppliers
SUPPLIER
DIODES
Coilcraft DO3316P-102HC
PHONE
FAX
Coilcraft
847-639-6400
847-639-1489
Coiltronics
561-241-7876
561-241-9339
Motorola
602-303-5454
602-994-6430
Panasonic
714-373-7939
714-373-7183
STMMicroelectronics
617-259-0300
617-259-9442
where I1 is the desired current limit in amperes, and
I LIM is the current limit value from the Electrical
Characteristics.
__________________Design Procedure
Inductor Selection (L1)
The MAX1709’s high switching frequency allows the
use of a small-size inductor. Use a 1.0µH inductor for
600kHz operation. If the MAX1709 will be synchronized
at a different frequency, scale the inductor value with
the inverse of frequency (L1 = 1µH ✕ 600kHz / fSYNC).
The PWM design tolerates inductor values within ±25%
of this calculated value, so choose the closest standard
inductor value. For example, use 1.5µH for 350kHz and
0.68µH for 1MHz).
Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for
use at high switching frequencies. Ensure the inductor’s saturation rating (the current at which the core
begins to saturate and inductance falls) exceeds the
internal current limit. Note that this current may be
reduced through SS/LIM if less than the MAX1709’s full
load current is needed (see Electrical Characteristics
for ratings). For highest efficiency, use a coil with low
DC resistance, preferably under 10mΩ. To minimize
radiated noise, use a toroid, pot core, or shielded
inductor. See Tables 3 and 4 for a list of recommended
components and component suppliers. To calculate
the maximum output current (in amperes), use the following equation:
V
+ VD − VIN
IOUT(MAX) = D' ILIM − D' OUT
2 × ƒ × L1
where:
VIN = input voltage
VD = forward voltage drop of the Schottky diode at ILIM
current
VOUT = output voltage
D' = (VIN) / (VOUT + VD), assuming switch voltage drop
is negligible
f = switching frequency
L1 = inductor value
VIN
FB
REF
SLOPE
COMP
LX
R
Q
N
VOUT
LX
S
MAX1709
R4
SS/LIM
12.5
(LIMITED TO 100mV)
OSCILLATOR
Figure 3. Simplified PWM Controller Functional Diagram
10
FB
11mΩ
PGND
KEEP SHORT
R3
Figure 4. Adjustable Output Voltage
______________________________________________________________________________________
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
µC
270k
MAX1709
ON/OFF
VDD
ONB
I/O
ONA
I/O
270k
0.1µF
MAX1709 IC Power Dissipation
The major components of MAX1709 dissipated power
are switch conductance loss (PSW), capacitive loss
(PCAP), and switch transition loss (PTRAN). Throughout
the formulas, numerical examples are provided in {},
corresponding to the following condition:
{VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 4A}
An important parameter to compute the power dissipated in the MAX1709 is the approximate peak switch current (ISW):
ILIM = minimum value of switch current limit from Electrical Characteristics or set by RSET/LIM.
I
ISW = OUT
D'
VIN
D' =
VOUT + VD
Diode Selection (D1)
PD (MAX1709) = PSW + PCAP + PTRAN
Figure 5. Momentary Pushbutton On-Off Switch
The MAX1709’s high switching frequency demands a
high-speed rectifier. Schottky diodes, such as the
MBRD1035CTL or STPS8L30B (Table 3), are recommended. The diode’s current rating must exceed the
maximum load current, and its breakdown voltage must
exceed VOUT. The diode must be placed within 10mm
of the LX switching node and the output filter capacitor.
The diode also must be able to dissipate the power calculated by the following equation:
PDIODE = IOUT ✕ VD
where IOUT is the average load current and VD is the
diode forward voltage at the peak switch current.
Capacitor Selection
Input Bypass Capacitors (C1, C2)
Two 150µF, low-ESR tantalum input capacitors will
reduce peak currents and reflected noise due to inductor current ripple. Lower ESR allows for lower input ripple current, but combined ESR values up to 50mΩ are
acceptable. Smaller ceramic capacitors may also be
used for light loads or in applications that can tolerate
higher input current ripple.
Output Filter Capacitors (C6, C7)
The output filter capacitor ESR must be kept under
15mΩ for stable operation. Two parallel 150µF polymer
capacitors (Panasonic EEFUE0J151R) typically exhibit
5mΩ of ESR. This translates to approximately 35mV of
output ripple at 7A switch current. Bypass the
MAX1709 IC supply input (OUT) with a 0.1µF ceramic
PSW = (1 - D') ISW2 ✕ RSW
{6.67A }
{0.6}
{0.83W}
{0.59W}
where:
RSW = switch resistance {33mΩ}
PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.09W}
CDIO = catch-diode capacitance {1000pF}
CDSW = switch drain capacitance {2500pF}
CGSW = switch gate capacitance {1500pF}
f = switching frequency {600kHz}
PTRAN = (VOUT + VD) ISW ✕ tSW ✕ f / 3
{0.15W}
where tSW = is switch turn-on or turn-off time {20ns}.
Applications Information
Using a Momentary On/Off Switch
A momentary pushbutton switch can be used to turn
the MAX1709 on and off. As shown in Figure 5, when
ONA is pulled low and ONB is pulled high, the part is
off. When the momentary switch is pressed, ONB is
pulled low and the regulator turns on. The switch
should be on long enough for the microcontroller to exit
reset. The controller issues a logic high to ONA, which
guarantees that the part will stay on regardless of the
subsequent switch state. To turn the regulator off,
press the switch long enough for the controller to read
the switch status and pull ONA low. When the switch is
released, ONB pulls high and the regulator turns off.
Layout Considerations
Due to high inductor current levels and fast switching
waveforms, proper PC board layout is essential. Protect
sensitive analog grounds by using a star ground config-
______________________________________________________________________________________
11
MAX1709
capacitor to GND and a 2Ω series resistor (R2, as
shown in Figure 1).
uration. Connect PGND, the input bypass capacitor
ground lead, and the output filter capacitor ground lead
to a single point (star ground configuration). In addition,
minimize trace lengths to reduce stray capacitance and
trace resistance, especially from the LX pins to the catch
diode (D1) and output capacitors (C6 and C7) to PGND
pins. If an external resistor-divider is used to set the output voltage (Figure 4), the trace from FB to the resistors
must be extremely short and must be shielded from
switching signals, such as CLK or LX. Refer to a layout
example in the MAX1709EVKIT data sheet.
___________________Chip Information
TRANSISTOR COUNT: 1112
Package Information
SOICN.EPS
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
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.
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is a registered trademark of Maxim Integrated Products.