19-2068; Rev 0; 7/01
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Features
♦ On-Chip 5A Power MOSFET
♦ 5V, 2A 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)
♦ Small QSOP Package
________________________Applications
Routers, Servers, Workstations, Card Racks
Local 2.5V to 3.3V or 5V Conversion
Ordering Information
PART
MAX1708EEE
TEMP. RANGE
PIN-PACKAGE
-40°C to +85°C
16 QSOP
Local 3.3V to 5V Conversion
3.6V or 5V RF PAs in Communications Handsets
Typical Operating Circuit
INPUT
1V TO 5V
TOP VIEW
2.2µH
OFF ON
SYNC
OR
INTERNAL
Pin Configuration
ONA
LX
MAX1708
CLK
16 CLK
ONA 2
15 3.3/5
LX 3
LX 4
14 PGND
MAX1708
LX 5
13 PGND
12 PGND
GND
GND 6
SS/LIM
REF
OUTPUT
3.3V, 5V,
OR ADJ
UP TO 2A
ONB 1
OUT
11 FB
SS/LIM 7
10 OUT
REF 8
9
GND
QSOP
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1708
General Description
The MAX1708 sets a new standard of space savings for
high-power, step-up DC-DC conversion. It delivers up
to 10W 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 push-button switch, as well as conventional on/off logic control.
The MAX1708 also features programmable soft-start
and current limit for design flexibility and optimum performance with batteries. The maximum RMS switch current rating is 5A. For a device with a higher (10A)
switch current rating, refer to the MAX1709 data sheet.
MAX1708
High-Frequency, High-Power, Low-Noise,
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 QSOP (derate 8.30mW/°C above +70°C). .......667mW
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
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
MAX
3.3/5 = GND, ISW = 0.5A
3.26
3.34
3.42
3.3/5 = OUT, ISW = 0.5A
4.90
5.05
5.20
-0.40
-0.60
1.215
1.240
1.265
V
1
200
nA
2.5
5.5
V
Rising and falling
2.0
2.3
V
Frequency in Startup Mode
VOUT =1.5V
40
400
kHz
Minimum Startup Voltage
IOUT < 1mA, TA = +25°C (Note 4)
0.9
Minimum Operating Voltage
(Note 5)
0.7
Soft-Start Pin Current
VSS/LIM = 1V
4
5.0
µA
OUT Supply Current
OUT Leakage Current In
Shutdown
LX Leakage Current
VFB = 1.5V (Note 6)
200
300
µA
V ONB = 3.6V
0.1
2
µA
1
25
µA
30
80
mΩ
Output Voltage
VFB < 0.1V (Note 1)
Load Regulation
Measured between 0.5A < ISW < 1.5A (Note 2)
FB Regulation Voltage (VFB)
ISW = 0.5A
FB Input Current
VFB = +1.5V
Output Voltage Adjust Range
Output Undervoltage Lockout
(Note 3)
3.2
VLX = V ONB = VOUT = +5.5V
N-Channel Switch
On-Resistance
N-Channel Current Limit
1.1
4.5
5.3
7.0
SS/LIM = 150kΩ to GND
1.80
3.00
3.85
RMS Switch Current
IREF = 0
-1µA ≤ IREF ≤ 50µA
Reference Supply Rejection
+2.5V ≤ VOUT ≤ +5.5V
Input Low Level (Note 7)
2
5
Reference Load Regulation
1.245
V
%/A
V
V
SS/LIM = open
Reference Voltage
UNITS
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
CLK, 2.7V < VOUT < 5.5V
0.2 ✕ VOUT
_______________________________________________________________________________________
✕
VOUT
V
High-Frequency, High-Power, Low-Noise,
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
Input High Level
CONDITIONS
0.8 ✕ VOUT
CLK, 2.7 V< VOUT < 5.5V
0.8 ✕ VOUT
ONA, ONB, CLK, 3.3/5 = 0, 5.5V
Logic Input Current
MIN
ONA, ONB, 3.3/5, 1.2V < VOUT <5.5V
TYP
MAX
UNITS
V
1
µA
Internal Oscillator Frequency
520
-1
600
680
kHz
Maximum Duty Cycle
External Clock Frequency
Range
CLK Pulse Width
82
88
94
%
1000
kHz
(Note 8)
CLK Rise/Fall Time
(Note 8)
350
100
ns
50
ns
UNITS
ELECTRICAL CHARACTERISTICS
(VOUT = VCLK = +3.6V, ONA = ONB = FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER
CONDITIONS
MIN
MAX
3.3/5 = GND, ISW = 0.5A
3.24
3.45
3.3/5 = OUT, ISW = 0.5A
4.90
5.22
1.20
1.28
V
200
nA
-0.60
%/A
5.2
µA
2
µA
300
µA
80
mΩ
Output Voltage
VFB < 0.1V, VIN = +2.4V
(Note 1)
FB Regulation Voltage
ISW = 0.5A
FB Input Current (VFB)
VFB = +1.5V
Load Regulation
Measured between 0.5A < ISW < 1.5A (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
SS/LIM = open
4.5
7.5
SS/LIM = 150kΩ to GND
1.8
4.0
IREF = 0
1.24
1.28
V
A
V
_______________________________________________________________________________________
3
MAX1708
ELECTRICAL CHARACTERISTICS (continued)
MAX1708
High-Frequency, High-Power, Low-Noise,
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
Input Low Level (Note 7)
Input High Level
Logic Input Current
CONDITIONS
MIN
MAX
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V
0.2 ✕ VOUT
CLK, 2.7V < VOUT < 5.5V
0.2 ✕ VOUT
ONA, ONB, 3.3/5, 1.2V < VOUT < 5.5V
0.8 ✕ VOUT
CLK, 2.7V < VOUT < 5.5V
0.8 ✕ VOUT
ONA, ONB, CLK, 3.3/5 = 0, 5.5V
UNITS
V
V
-1
1
µA
Internal Oscillator Frequency
500
700
kHz
Maximum Duty Cycle
80
95
%
350
1000
kHz
External Clock Frequency Range
CLK Pulse Width
(Note 8)
CLK Rise/Fall Time
(Note 8)
100
ns
50
ns
Note 1: Output voltage is specified at 0.5A switch current ISW, which is equivalent to approximately 0.5A ✕ (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 MAX1708 is bootstrapped and will operate down to a 0.7V input once started.
Note 6: Supply current is measured from the output voltage (+3.3V) to the OUT pin. 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
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
VIN = 2.5V
80
EFFICIENCY vs. SWITCHING FREQUENCY
90
MAX1708 toc01b
MAX1708 toc01a
VIN = 3.3V
80
EFFICIENCY vs. OUTPUT CURRENT
100
MAX1708 toc02
EFFICIENCY vs. OUTPUT CURRENT
100
60
40
60
VIN = 1.2V
40
20
VOUT = 5V
VOUT = 3.3V
1
10
100
1000
0.1
450
550
650
750
850
0.5
0
-0.5
-1.0
-1.5
-1.5
10
100
0.1
0
-0.1
IOUT = 1A
1000
IOUT = 500mA
-0.2
PLOT NORMALIZED TO VIN = 3.3V
-2.0
-0.3
0.1
10,000
1
10
1000
100
10,000
2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
LINE REGULATION
(VOUT = 3.3V)
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
INPUT CURRENT (mA)
0.5
0.4
0.3
0.2
0.1
VOUT = 5V, VIN INCREASING
15
10
VOUT = 3.3V, VIN INCREASING
0
5
-0.1
IOUT = 500mA
PLOT NORMALIZED TO VIN = 2.5V
2.00
2.25
2.50
INPUT VOLTAGE (V)
2.75
60
VOUT = 5V, VIN DECREASING
50
40
30
20
VOUT = 3.3V, VIN DECREASING
10
0
0
3.00
MAX1708 toc05b
20
INPUT CURRENT (mA)
IOUT = 1A
70
MAX1708 toc05a
25
MAX1708 toc04b
0.8
1.75
0.2
PLOT NORMALIZED TO IOUT = 500mA
PLOT NORMALIZED TO IOUT = 500mA
-2.0
1
MAX1708 toc04a
1.0
VOUT REGULATION (%)
VOUT REGULATION (%)
1.5
950
0.3
MAX1708 toc03b
MAX1708 toc03a
2.0
-1.0
1.50
350
LINE REGULATION
(VOUT = 5V)
-0.5
-0.3
10,000
LOAD REGULATION
(VIN = 2.5V, VOUT = 3.3V)
0
0.6
1000
100
LOAD REGULATION
(VIN = 3.3V, VOUT = 5V)
0.5
0.7
10
SWITCHING FREQUENCY (kHz)
1.0
0.1
1
OUTPUT CURRENT (mA)
1.5
VOUT REGULATION (%)
10,000
86
OUTPUT CURRENT (mA)
2.0
VOUT REGULATION (%)
VIN = 3.3V, VOUT = 5V, IOUT =1A
0
0.1
88
87
20
0
-0.2
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
89
VIN = 2.5V
0
1
2
3
4
INPUT VOLTAGE (V)
5
6
0
1
2
3
4
5
6
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
MAX1708
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
STARTUP VOLTAGE vs. LOAD CURRENT
(VOUT = 3.3V)
TA = -40°C
1.0
TA = +25°C
0.8
TA = +85°C
0.6
0.4
1.5
TA = -40°C
TA = +25°C
1.0
TA = +85°C
0.5
PLOT NORMALIZED TO 25°C
FREQUENCY CHANGE (%)
1.2
2.0
STARTUP VOLTAGE (V)
1.4
2
MAX1708 toc06b
1.6
STARTUP VOLTAGE (V)
2.5
MAX1708 toc06a
1.8
SWITCHING FREQUENCY
vs. TEMPERATURE
1
0
-1
VOUT = 3.3V
0.2
0
10
100
1000
LOAD CURRENT (mA)
1
10,000
NOISE vs. FREQUENCY
SWITCH CURRENT LIMIT (A)
RESOLUTION = 1kHz
3000
2500
2000
1500
1000
1000
10,000
-40
-15
10
35
60
TEMPERATURE (°C)
SWITCH CURRENT LIMIT
vs. SS/LIM RESISTANCE
SWITCH CURRENT LIMIT
vs. TEMPERATURE
MAX1708 toc09
3500
100
LOAD CURRENT (mA)
5
MAX1708 toc08
4000
10
4
3
2
1
6.0
85
MAX1708 toc10
1
-2
SWITCH CURRENT LIMIT (A)
0
5.5
5.0
4.5
500
VIN = 3.3V, VOUT = 5V
0
0
0.1
1
10
4.0
0
FREQUENCY (MHz)
50
100
150
200
250
300
-40
10
LINE TRANSIENT RESPONSE
MAX1708 toc12
3.5V
VIN
500mV/div 3V
5V
0
IL 4A
2A/div
2A
VOUT 5V
AC-COUPLED
50mV/div
0
VOUT 5V
AC-COUPLED
50mV/div
1µs/div
35
TEMPERATURE (°C)
MAX1708 toc11
VLX
5V/div
-15
SS/LIM RESISTANCE (kΩ)
HEAVY SWITCHING WAVEFORM
6
MAX1708 toc07
STARTUP VOLTAGE vs. LOAD CURRENT
(VOUT = 5V)
NOISE (µV)
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
100µs/div
_______________________________________________________________________________________
60
85
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
STARTUP WITHOUT SOFT-START
(CSS = 0)
LOAD TRANSIENT RESPONSE
MAX1708 toc14
MAX1708 toc13
5V
VONA
5V/div 0
4A
ISW
2A/div 2A
0
VOUT 5V
AC-COUPLED
50mV/div 0
IIN 2A
1A/div
1A
2A
0
IOUT 1A
1A/div
0
VOUT 4V
2V/div
2V
40µs/div
2ms/div
STARTUP WITH SOFT-START
(CSS = 0.01µF)
STARTUP WITH SOFT-START
(CSS = 0.1µF)
MAX1708 toc15
5V
VONA
5V/div 0
IIN
1A/div
IIN 1A
1A/div
0
1A
0
4V
4V
2V
MAX1708 toc16
5V
VONA
5V/div 0
2A
VOUT
2V/div
RL = 5Ω
VOUT 2V
2V/div
0
RL = 5Ω
RL = 5Ω
0
2ms/div
2ms/div
_______________________________________________________________________________________
7
MAX1708
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Pin Description
8
PIN
NAME
1
ONB
2
ONA
3, 4, 5
LX
6, 9
GND
FUNCTION
Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).
On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1).
Drain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect
an external Schottky diode from LX to VOUT. (Figure 1)
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, SS/LIM is internally pulled to GND to discharge the soft-start capacitor.
7
SS/LIM
8
REF
Voltage Reference Output. Bypass with a 0.22µF capacitor to GND. Maximum REF load is
50µA.
10
OUT
Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1µF capacitor
to PGND with less than 5mm trace length. Connect a 2Ω series resistor from the output filter
capacitor (0.1µF) to OUT (Figure 1).
11
FB
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 +1.24V (Figure 4).
12, 13, 14
PGND
15
3.3/5
Output Voltage Selection Input. 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.
16
CLK
Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK with
an external clock for external synchronization.
Power Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14
together with wide traces.
_______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
D1
VOUT
+5V
VIN
KEEP TRACES
SHORT AND WIDE
C1
150µF
GND
MAX1708
L1
2.2µH
LX
LX
LX
R2
2Ω
ONA
ONB
ON/OFF
CONTROL
C2
150µF
GND
CLK
R1
3.3/5
C3
MAX1708
SS/LIM
OUT
C5
0.1µF
C4
0.22µF
FB
REF
GND
KEEP TRACES
SHORT AND WIDE
GND
PGND PGND PGND
Figure 1. Standard Operating Circuit
_______________Detailed Description
The MAX1708 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 MAX1708 internal N-channel MOSFET switch is
rated for 5A (RMS value) and can deliver loads to 2A,
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 1MHz.
ONA, ONB
The logic levels at ONA and ONB turn the MAX1708 on
or off. When ONA = 1 or ONB = 0, the device is on.
When ONA = 0 and ONB = 1, the device is off (Table
1). Logic high on-control can be implemented by connecting ONB high and using ONA for shutdown.
Implement inverted single-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 MAX1708 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 MAX1708 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 MAX1708 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
MAX1708. Figure 3 shows the simplified PWM con-
_______________________________________________________________________________________
9
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
Table 1. On/Off Logic Control
ONB
0
ONA
0
Table 2. Selecting Switching Frequency
MAX1708
On
0
1
Off
1
0
On
1
1
On
CLK
0
1
External clock
(350kHz−1000kHz)
MAX1708
IC POWER
PWM
CONTROLLER
2.15V
EN
ONA
1.26V
DUAL MODE
FB
STARTUP
Q
OSCILLATOR
D
EN
ON
RDY
REFERENCE
EN
CLK
FB
3.3/5
GND
Synchronized PWM
UNDERVOLTAGE LOCKOUT
OUT
ONB
REF
MODE
Not allowed
PWM
600kHz
OSCILLATOR
LX
OSC
N
PGND
FB
OUT
Figure 2. Simplified Functional Diagram
troller functional diagram. The MAX1708 enters synchronized current-mode PWM when a clock signal
(350kHz < fCLK < 1MHz) 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 higher frequencies (see MAX1708 IC
Power Dissipation).
Setting the Output Voltage
The MAX1708 features Dual Mode™ operation. When
FB is connected to ground, the MAX1708 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
Dual Mode is a trademark of Maxim Integrated Products.
10
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 GND.
Select capacitor C3 (see Figure 1):
tSS = 4ms + [110 ✕ C3 (in µF)]
where tSS is the time (in milliseconds) it takes output to
reach its final 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 = 312kΩ ×
I1
ILIM
where I1 is the desired current limit in amperes and R1
≤ 312kΩ. ILIM = 5A, if R1 is omitted.
______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
PRODUCTION
Surface mount
INDUCTORS
CAPACITORS
Sanyo 6TPC100M
Motorola MBRD1035CTL
Coilcraft DO3316P-222HC
Panasonic EEFUE0J151R
Central CMSH5-20
Table 4. Component Suppliers
SUPPLIER
Central
DIODES
Coiltronics UP2B-2R2
PHONE
FAX
631-435-1110
631-435-1824
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
__________________Design Procedure
Inductor Selection (L1)
The MAX1708’s high switching frequency allows the
use of a small-size inductor. Use a 2.2µH inductor for
600kHz operation. If the MAX1708 is synchronized at a
different frequency, scale the inductor value with the
inverse of frequency (L1 = 2.2µH ✕ 600kHz / fSYNC).
The PWM design tolerates inductor values within ±25%
of this calculated value, so choose the closest standard
inductor value. For example, use 3.3µH for 350kHz and
1.5µH 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 MAX1708’s full
load current is needed (see Electrical Characteristics
for ratings). For highest efficiency, use a coil with low
DC resistance, preferably under 20mΩ. 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
× ƒ × L1
2
where:
VIN = input voltage
VD = forward voltage drop of the Schottky diode
at ILIM
VOUT = output voltage
D' = (VIN) / (VOUT + VD), neglecting switch voltage
drop
f = switching frequency
L1 = inductor value
ILIM = minimum value of switch current limit from
Electrical Characteristics or set by R1 of
Figure 1.
VIN
FB
REF
SLOPE
COMP
LX
R
Q
N
VOUT
LX
S
MAX1708
R4
SS/LIM
12.5
(LIMITED TO 100mV)
FB
11mΩ
PGND
OSCILLATOR
Figure 3. Simplified PWM Controller Functional Diagram
KEEP SHORT
R3
Figure 4. Adjustable Output Voltage
______________________________________________________________________________________
11
MAX1708
Table 3. Component Selection Guide
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
MAX1708
MAX1708 IC Power Dissipation
µC
270kΩ
MAX1708
ON/OFF
ONB
VDD
I/O
ONA
I/O
270kΩ
0.1µF
Figure 5. Momentary Pushbutton On-Off Switch
Diode Selection (D1)
The MAX1708’s high switching frequency demands a
high-speed rectifier. Use Schottky diodes (Table 3).
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 Capacitor (C1)
A 150µF, low-ESR input capacitor 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 100mΩ are acceptable.
Smaller ceramic capacitors may also be used for light
loads or in applications that can tolerate higher input
current ripple.
Output Filter Capacitor (C2)
The output filter capacitor ESR must be kept under
30mΩ for stable operation. Polymer capacitors of
150µF (Panasonic EEFUE0J151R) typically exhibit
10mΩ of ESR. This translates to approximately 35mV of
output ripple at 3.5A switch current. Bypass the
MAX1708 IC supply input (OUT) with a 0.1µF ceramic
capacitor to GND and a 2Ω series resistor (R2, as
shown in Figure 1).
The major components of MAX1708 dissipated power
are switch conductance loss (PSW), capacitive loss
(PCAP), and switch transition loss (PTRAN). Numerical
examples provided in brackets ({ }) correspond to the
following condition:
{VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 2A}
An important parameter to compute the power dissipated in the MAX1708 is the approximate peak switch current (ISW):
I
ISW = OUT {3.33A }
D'
VIN
{0.6}
D' =
VOUT + VD
PD = PSW + PCAP + PTRAN {0.472W}
PSW = (1 - D') ISW2 ✕ RSW {0.353W}
PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.045W}
PTRAN = (VOUT + VD) ISW ✕ tSW ✕ f / 3 {0.073W}
where:
RSW = switch resistance {80mΩ}
CDIO = catch-diode capacitance {500pF}
CDSW = switch drain capacitance {1250pF}
CGSW = switch gate capacitance {750pF}
f = switching frequency {600kHz}
tSW = 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 MAX1708 on and off. As shown in Figure 5, when
ONA is pulled low and ONB is pulled high, the device
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 device will stay on regardless of
the subsequent switch state. To turn the regulator off,
depress 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
12
______________________________________________________________________________________
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
accomplished with a large PGND plane on the surface of
the board. Also note that outer-layer ground plane area
beneath the device provides little heat-sinking benefit. If
an outer-layer ground plane is not feasible, the PGND
pins should be connected to the inner-layer ground
plane with multiple vias (at least three vias per pin is recommended). Since the purpose of these vias is to optimize thermal conductivity to the inner ground plane, be
sure that the vias have no gaps in their connections to
the ground plane. Refer to a layout example in the
MAX1708EVKIT data sheet.
___________________ Chip Information
TRANSISTOR COUNT: 1112
SUBSTRATE: GND
PROCESS: BiCMOS
QSOP.EPS
Package Information
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.
13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2001 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX1708
sensitive analog grounds by using a star ground configuration. 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 capacitor (C2) 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. To optimize package power
dissipation and minimize device heating under heavy
loads, expand PC trace area connected to the three
PGND pins as much as the layout can allow. This is best