Page 1
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.
________________________Applications
Routers, Servers, Workstations, Card Racks
Local 2.5V to 3.3V or 5V Conversion
Local 3.3V to 5V Conversion
3.6V or 5V RF PAs in Communications Handsets
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
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
________________________________________________________________ Maxim Integrated Products 1
Pin Configuration
ONA
REF
SS/LIM
CLK
OUTPUT
3.3V, 5V,
OR ADJ
UP TO 2A
OUT
GND
LX
2.2µH
OFF
ON
INPUT
1V TO 5V
SYNC
OR
INTERNAL
MAX1708
Typical Operating Circuit
19-2068; Rev 0; 7/01
Ordering Information
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.
PART TEMP. RANGE PIN-PACKAGE
MAX1708EEE -40°C to +85°C 16 QSOP
TOP VIEW
1
ONB CLK
ONA
2
LX
3
4
LX
LX
5
GND
6
SS/LIM
REF
7
8
MAX1708
QSOP
16
15
3.3/5
14
PGND
13
PGND
12
PGND
FB
11
10
OUT
9
GND
Page 2
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V
OUT
= V
CLK
= +3.6V, ONA = ONB = FB = 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.
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 (V
OUT
+ 0.3V)
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (T
A
= +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
Output Voltage VFB < 0.1V (Note 1)
Load Regulation Measured between 0.5A < ISW < 1.5A (Note 2) -0.40 -0.60 %/A
FB Regulation Voltage (VFB)ISW = 0.5A 1.215 1.240 1.265 V
FB Input Current V
Output Voltage Adjust Range 2.5 5.5 V
Output Undervoltage Lockout
(Note 3)
Frequency in Startup Mode V
Minimum Startup Voltage I
Minimum Operating Voltage (Note 5) 0.7 V
Soft-Start Pin Current V
OUT Supply Current VFB = 1.5V (Note 6) 200 300 µA
OUT Leakage Current In
Shutdown
LX Leakage Current VLX = V
N-Channel Switch
On-Resistance
N-Channel Current Limit
RMS Switch Current 5A
Reference Voltage I
Reference Load Regulation -1µA ≤ I
Reference Supply Rejection +2.5V ≤ V
Input Low Level (Note 7)
PARAMETER CONDITIONS MIN TYP MAX UNITS
= +1.5V
FB
Rising and falling 2.0 2.3 V
=1.5V 40 400 kHz
OUT
< 1mA, TA = +25°C (Note 4) 0.9 1.1 V
OUT
= 1V 3.2 4 5.0 µA
SS/LIM
V
= 3.6V 0.1 2 µA
ONB
= V
ONB
SS/LIM = open
SS/LIM = 150kΩ to GND
= 0 1.245 1.260 1.275 V
REF
≤ 50µA 4 10 mV
REF
OUT
ONA, ONB, 3.3/5, 1.2V < V
CLK, 2.7V < V
= +5.5V 1 25 µA
OUT
≤ +5.5V 0.2 5 mV
< 5.5V 0.2 ✕ V
OUT
3.3/5 = GND, ISW = 0.5A 3.26 3.34 3.42
3.3/5 = OUT, I
< 5.5V 0.2 ✕ V
OUT
= 0.5A 4.90 5.05 5.20
SW
1
30 80 mΩ
4.5
1.80
5.3
3.00
200 nA
7.0
3.85
OUT
OUT
V
A
RMS
V
Page 3
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(V
OUT
= V
CLK
= +3.6V, ONA = ONB = FB = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
ELECTRICAL CHARACTERISTICS
(V
OUT
= V
CLK
= +3.6V, ONA = ONB = FB = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 9)
Input High Level
Logic Input Current ONA, ONB, CLK, 3.3/5 = 0, 5.5V -1 1 µA
Internal Oscillator Frequency 520 600 680 kHz
Maximum Duty Cycle 82 88 94 %
External Clock Frequency
Range
CLK Pulse Width (Note 8) 100 ns
CLK Rise/Fall Time (Note 8) 50 ns
PARAMETER CONDITIONS MIN TYP MAX UNITS
ONA, ONB, 3.3/5, 1.2V < V
CLK, 2.7 V< V
< 5.5V 0.8 ✕ V
OUT
<5.5V 0.8 ✕ V
OUT
OUT
OUT
350 1000 kHz
Output Voltage
FB Regulation Voltage I
FB Input Current (VFB)V
Load Regulation Measured between 0.5A < ISW < 1.5A (Note 2) -0.60 %/A
Soft-Start Pin Current SS/LIM = 1V 3.2 5.2 µA
OUT Leakage Current in
Shutdown
OUT Supply Current V
N-Channel Switch
On-Resistance
N-Channel Current Limit
Reference Voltage I
PARAMETER CONDITIONS MIN MAX UNITS
V
< 0.1V, VIN = +2.4V
FB
(Note 1)
= 0.5A 1.20 1.28 V
SW
= +1.5V 200 nA
FB
V
= 3.6V 2 µA
ONB
= 1.5V (Note 6) 300 µA
FB
SS/LIM = open 4.5 7.5
SS/LIM = 150kΩ to GND 1.8 4.0
= 0 1.24 1.28 V
REF
3.3/5 = GND, I
3.3/5 = OUT, I
= 0.5A 3.24 3.45
SW
= 0.5A 4.90 5.22
SW
80 mΩ
V
V
A
Page 4
Note 1: Output voltage is specified at 0.5A switch current ISW, which is equivalent to approximately 0.5A ✕(VIN/ V
OUT
) of load cur-
rent.
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 I
SW
✕
(VIN/ V
OUT
).
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.
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(V
OUT
= V
CLK
= +3.6V, ONA = ONB = FB = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 9)
Input Low Level (Note 7)
Input High Level
Logic Input Current ONA, ONB, CLK, 3.3/5 = 0, 5.5V -1 1 µA
Internal Oscillator Frequency 500 700 kHz
Maximum Duty Cycle 80 95 %
External Clock Frequency Range 350 1000 kHz
CLK Pulse Width (Note 8) 100 ns
CLK Rise/Fall Time (Note 8) 50 ns
PARAMETER CONDITIONS MIN MAX UNITS
ONA, ONB, 3.3/5, 1.2V < V
CLK, 2.7V < V
ONA, ONB, 3.3/5, 1.2V < V
CLK, 2.7V < V
< 5.5V 0.2 ✕ V
OUT
< 5.5V 0.8 ✕ V
OUT
< 5.5V 0.2 ✕ V
OUT
< 5.5V 0.8 ✕ V
OUT
OUT
OUT
OUT
OUT
V
V
Page 5
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
_______________________________________________________________________________________ 5
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
100
0
0.1 1 1000 10,000
EFFICIENCY vs. OUTPUT CURRENT
MAX1708 toc01a
OUTPUT CURRENT (mA)
EFFICIENCY (%)
10 100
20
40
60
80
VIN = 3.3V
VIN = 2.5V
V
OUT
= 5V
2.0
-2.0
0.1 1 1000 10,000
LOAD REGULATION
(V
IN
= 3.3V, V
OUT
= 5V)
-1.5
0.5
1.0
1.5
MAX1708 toc03a
OUTPUT CURRENT (mA)
V
OUT
REGULATION (%)
10 100
-1.0
-0.5
0
PLOT NORMALIZED TO I
OUT
= 500mA
2.0
-2.0
0.1 1 1000 10,000
LOAD REGULATION
(V
IN
= 2.5V, V
OUT
= 3.3V)
-1.5
0.5
1.0
1.5
MAX1708 toc03b
OUTPUT CURRENT (mA)
V
OUT
REGULATION (%)
10 100
-1.0
-0.5
0
PLOT NORMALIZED TO I
OUT
= 500mA
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.50 1.75 2.00 2.25 2.50 2.75 3.00
LINE REGULATION
(V
OUT
= 3.3V)
MAX1708 toc04b
INPUT VOLTAGE (V)
V
OUT
REGULATION (%)
PLOT NORMALIZED TO VIN = 2.5V
I
OUT
= 500mA
I
OUT
= 1A
0
5
15
10
20
25
0213456
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
MAX1708 toc05a
INPUT VOLTAGE (V)
INPUT CURRENT (mA)
V
OUT
= 5V, VIN INCREASING
V
OUT
= 3.3V, VIN INCREASING
0
20
10
40
30
60
50
70
0231 456
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
MAX1708 toc05b
INPUT VOLTAGE (V)
INPUT CURRENT (mA)
V
OUT
= 3.3V, VIN DECREASING
V
OUT
= 5V, VIN DECREASING
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
2.00 2.502.25 2.75 3.00 3.25 3.50 3.75 4.00
LINE REGULATION
(V
OUT
= 5V)
MAX1708 toc04a
INPUT VOLTAGE (V)
V
OUT
REGULATION (%)
PLOT NORMALIZED TO VIN = 3.3V
I
OUT
= 500mA
I
OUT
= 1A
100
0
0.1 1 1000 10,000
EFFICIENCY vs. OUTPUT CURRENT
MAX1708 toc01b
OUTPUT CURRENT (mA)
EFFICIENCY (%)
10 100
20
40
60
80
VIN = 1.2V
VIN = 2.5V
V
OUT
= 3.3V
86
87
88
89
90
EFFICIENCY vs. SWITCHING FREQUENCY
MAX1708 toc02
SWITCHING FREQUENCY (kHz)
EFFICIENCY (%)
350 650 750450 550 850 950
VIN = 3.3V, V
OUT
= 5V, I
OUT
=1A
Page 6
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
STARTUP VOLTAGE vs. LOAD CURRENT
(V
1.8
1.6
1.4
TA = -40°C
1.2
= +25°C
T
1.0
A
TA = +85°C
0.8
0.6
STARTUP VOLTAGE (V)
0.4
0.2
0
1 10 100
LOAD CURRENT (mA)
NOISE vs. FREQUENCY
4000
3500
3000
2500
2000
NOISE (µV)
1500
1000
500
0
0.1 1 10
FREQUENCY (MHz)
= 5V)
OUT
RESOLUTION = 1kHz
1000 10,000
STARTUP VOLTAGE vs. LOAD CURRENT
2.5
2.0
MAX1708 toc06a
1.5
TA = -40°C
TA = +25°C
1.0
STARTUP VOLTAGE (V)
TA = +85°C
0.5
0
1 100 100010 10,000
5
MAX1708 toc08
4
3
2
SWITCH CURRENT LIMIT (A)
1
0
= 3.3V)
(V
OUT
LOAD CURRENT (mA)
SWITCH CURRENT LIMIT
vs. SS/LIM RESISTANCE
500 250200150100 300
SS/LIM RESISTANCE (kΩ)
2
MAX1708 toc06b
MAX1708 toc09
1
0
FREQUENCY CHANGE (%)
-1
-2
-40 10-15 35 60 85
6.0
5.5
5.0
SWITCH CURRENT LIMIT (A)
4.5
4.0
-40 10-15 35 60 85
SWITCHING FREQUENCY
vs. TEMPERATURE
PLOT NORMALIZED TO 25°C
V
= 3.3V
OUT
TEMPERATURE (°C)
SWITCH CURRENT LIMIT
vs. TEMPERATURE
VIN = 3.3V, V
= 5V
OUT
TEMPERATURE (°C)
MAX1708 toc07
MAX1708 toc10
LINE TRANSIENT RESPONSE
3V
5V
MAX1708 toc12
100µs/div
V
5V/div
2A/div
V
OUT
AC-COUPLED
50mV/div
HEAVY SWITCHING WAVEFORM
5V
LX
0
4A
I
L
2A
0
5V
1µs/div
MAX1708 toc11
V
500mV/div
V
OUT
AC-COUPLED
50mV/div
IN
3.5V
Page 7
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
_______________________________________________________________________________________ 7
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
40µs/div
LOAD TRANSIENT RESPONSE
I
OUT
1A/div
V
OUT
AC-COUPLED
50mV/div
4A
5V
2A
1A
2A
0
0
0
I
SW
2A/div
MAX1708 toc13
2ms/div
STARTUP WITHOUT SOFT-START
(C
SS
= 0)
V
OUT
2V/div
I
IN
1A/div
5V
2A
1A
0
4V
0
2V
V
ONA
5V/div
MAX1708 toc14
RL = 5
Ω
2ms/div
STARTUP WITH SOFT-START
(C
SS
= 0.01µF)
V
OUT
2V/div
I
IN
1A/div
5V
1A
0
2A
4V
2V
0
0
V
ONA
5V/div
MAX1708 toc15
RL = 5
Ω
2ms/div
STARTUP WITH SOFT-START
(C
SS
= 0.1µF)
V
OUT
2V/div
I
IN
1A/div
5V
1A
0
4V
2V
0
0
V
ONA
5V/div
MAX1708 toc16
RL = 5
Ω
Page 8
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 ONB Shutdown Input. When ONB = high and ONA = low, the device turns off (Table 1).
2 ONA On-Control Input. When ONA = high or ONB = low, the device turns on (Table 1).
3, 4, 5 LX
6, 9 GND Ground
7 SS/LIM
8 REF
10 OUT
11 FB
12, 13, 14 PGND
15 3.3/5
16 CLK
Drain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect
an external Schottky diode from LX to V
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.
Voltage Reference Output. Bypass with a 0.22µF capacitor to GND. Maximum REF load is
50µA.
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).
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).
Power Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14
together with wide traces.
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.
Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK with
an external clock for external synchronization.
. (Figure 1)
OUT
Page 9
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
_______________________________________________________________________________________ 9
_______________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-
Figure 1. Standard Operating Circuit
V
IN
GND
KEEP TRACES
SHORT AND WIDE
C1
150µF
L1
2.2µH
KEEP TRACES
SHORT AND WIDE
ON/OFF
CONTROL
R1
C3
C4
0.22µF
ONA
ONB
SS/LIM
REF
GND
LX
LX
MAX1708
PGND PGND
LX
PGND
D1
CLK
3.3/5
OUT
GND
V
OUT
+5V
R2
2Ω
C5
0.1µF
FB
C2
150µF
GND
Page 10
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
10 ______________________________________________________________________________________
troller functional diagram. The MAX1708 enters synchronized current-mode PWM when a clock signal
(350kHz < f
CLK
< 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
where V
FB
= 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 tSSis 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:
where I1is the desired current limit in amperes and R1
≤ 312kΩ. I
LIM
= 5A, if R1 is omitted.
Figure 2. Simplified Functional Diagram
Table 2. Selecting Switching FrequencyTable 1. On/Off Logic Control
Dual Mode is a trademark of Maxim Integrated Products.
ONA ONB MAX1708
00On
0 1 Off
10On
11On
OUT
IC POWER
2.15V
ONA
ONB
REF
CLK
FB
3.3/5
GND
1.26V
UNDERVOLTAGE LOCKOUT
ON
RDY
REFERENCE
DUAL MODE
FB
OUT
MAX1708
STARTUP
EN Q
OSCILLATOR
EN
600kHz
OSCILLATOR
CLK MODE
0 Not allowed
1 PWM
External clock
(350kHz−1000kHz)
PWM
CONTROLLER
D
EN
OSC
FB
V
OUT
V
FB
RR
34 1 =−
Synchronized PWM
LX
N
PGND
I
Rk
1 312
=×Ω
I
LIM
1
Page 11
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
______________________________________________________________________________________ 11
__________________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 (L
1
= 2.2µH ✕600kHz / f
SYNC
).
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:
where:
V
IN
= input voltage
VD= forward voltage drop of the Schottky diode
at I
LIM
V
OUT
= output voltage
D' = (VIN) / (V
OUT
+ VD), neglecting switch voltage
drop
f = switching frequency
L1 = inductor value
I
LIM
= minimum value of switch current limit from
Electrical Characteristics or set by R1 of
Figure 1.
Table 4. Component Suppliers
Table 3. Component Selection Guide
Figure 3. Simplified PWM Controller Functional Diagram
Figure 4. Adjustable Output Voltage
PRODUCTION INDUCTORS CAPACITORS DIODES
Surface mount
SUPPLIER PHONE FAX
Central 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
Coiltronics UP2B-2R2 Sanyo 6TPC100M Motorola MBRD1035CTL
Coilcraft DO3316P-222HC Panasonic EEFUE0J151R Central CMSH5-20
IDID
OUT MAX LIM
()
''=−
V
IN
FB
SLOPE
COMP
REF
Q
R
S
LX
N
LX
MAX1708
L
OUT
VVV
+−
OUT D IN
׃×
21
V
SS/LIM
(LIMITED TO 100mV)
12.5
OSCILLATOR
11mΩ
PGND
FB
KEEP SHORT
R3
R4
Page 12
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
12 ______________________________________________________________________________________
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
V
OUT
. 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:
P
DIODE
= I
OUT
✕
V
D
where I
OUT
is the average load current and VDis 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).
MAX1708 IC Power Dissipation
The major components of MAX1708 dissipated power
are switch conductance loss (PSW), capacitive loss
(P
CAP
), and switch transition loss (P
TRAN
). Numerical
examples provided in brackets ({ }) correspond to the
following condition:
{VIN= 3.3V, V
OUT
= 5V, VD= 0.5V, I
OUT
= 2A}
An important parameter to compute the power dissipated in the MAX1708 is the approximate peak switch current (ISW):
PD= PSW+ P
CAP
+ P
TRAN
{0.472W}
PSW= (1 - D') I
SW
2
✕
RSW{0.353W}
P
CAP
= (C
DIO
+ C
DSW
+ C
GSW
) (V
OUT
+ VD)2f {0.045W}
P
TRAN
= (V
OUT
+ VD) I
SW
✕
t
SW
✕
f / 3 {0.073W}
where:
RSW= switch resistance {80mΩ}
C
DIO
= catch-diode capacitance {500pF}
C
DSW
= switch drain capacitance {1250pF}
C
GSW
= 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
Figure 5. Momentary Pushbutton On-Off Switch
270kΩ
ON/OFF
MAX1708
ONB
ONA
0.1µF
270kΩ
µC
V
DD
I/O
I/O
I
I
SW
'
OUT
' {. }
=
D
V
=
D
IN
VV
OUT D
333
+
A
06
{.}
Page 13
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.
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
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.
Package Information
___________________ Chip Information
TRANSISTOR COUNT: 1112
SUBSTRATE: GND
PROCESS: BiCMOS
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
MAX1708
QSOP.EPS
Loading...