1-Cell to 3-Cell, High-Power (1A),
Low-Noise, Step-Up DC-DC Converters
MAX1700/MAX1701
0.7V TO 5.5V
POUT
22µF
L1
10µH
MBR0520L
OUT
FB
REF
P
R
Q
N
LX
MAX1700
MAX1701
CLK/SEL
ONA
ONB
LX
D1
POUT
10Ω
OUT
0.22µF
2x
100µF
0.22µF
OUTPUT
S
R1
REF
0.22µF
GND
FB
PGND
FIXED
OUTPUT
(GND)
ADJUSTABLE
R2
1.3A CURRENT
LIMIT
OSC
PGND
Figure 2. Fixed or Adjustable Output (PWM mode).
Figure 3. Simplified PWM Controller Block Diagram
Table 1. Typical Available Output Current
NUMBER
OF CELLS
1 NiCd/NiMH
2 NiCd/NiMH
2 NiCd/NiMH
3 NiCd/NiMH
or 1 Li-Ion
INPUT
OUTPUT
OUTPUT
VOLTAGE (V) VOLTAGE (V) CURRENT (mA)
1.2
2.4
2.4
3.6
3.3
3.3
5.0
5.0
300
750
525
850
the output filter capacitor and load. As the energy
stored in the inductor is depleted, the current ramps
down and the output diode and synchronous rectifier
turn off. Voltage across the load is regulated using
either low-noise PWM or low-power operation, depend-
ing on the CLK/SEL pin setting (Table 2).
Table 2. Selecting the Operating Mode
CLK/SEL
0
1
External Clock
(200kHz to 400kHz)
MODE
Low Power
PWM
Synchronized
PWM
FEATURES
Low supply current
Low noise,
high output current
Low noise,
high output current
Step-Up Converter
The step-up switching DC-DC converter generates an
adjustable output from 2.5V to 5.5V. During the first part
of each cycle, the internal N-channel MOSFET switch is
turned on. This allows current to ramp up in the induc-
tor and store energy in a magnetic field. During the
second part of each cycle, when the MOSFET is turned
off, the voltage across the inductor reverses and forces
current through the diode and synchronous rectifier to
Low-Noise PWM Operation
When CLK/SEL is pulled high, the MAX1700/MAX1701
operate in a higher power, low-noise pulse-width-
modulation (PWM) mode. During PWM operation, they
switch at a constant frequency (300kHz) and then mod-
ulate the MOSFET switch pulse width to control the
power transferred per cycle and regulate the voltage
across the load. In PWM mode the devices can output
up to 800mA. Switching harmonics generated by fixed-
frequency operation are consistent and easily filtered.
See the Noise Spectrum Plot in the
Typical Operating
Characteristics.
During PWM operation, each rising edge of the internal
clock sets a flip-flop, which turns on the N-channel
MOSFET switch (Figure 3). The switch is turned off
when the sum of the voltage-error, slope compensation,
and current-feedback signals trips a multi-input com-
parator and resets the flip-flop; the switch remains off
for the rest of the cycle. When a change occurs in the
output-voltage error signal into the comparator, it shifts
the level to which the inductor current is allowed to
ramp during each cycle and modulates the MOSFET
switch pulse width. A second comparator enforces an
inductor current limit of 1.6A max.
10
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