Synchronized PWM Operation

The MAX1703 can be synchronized in PWM mode to a

200kHz to 400kHz frequency by applying an external

clock to CLK/SEL. This allows the user to set the har-

monics to avoid IF bands in wireless applications. The

synchronous rectifier is also active during synchronized

PWM operation.

Low-Power PFM Operation

Pulling CLK/SEL low places the MAX1703 in a low-

power mode. During low-power mode, PFM operation

regulates the output voltage by transferring a fixed

amount of energy during each cycle, and then modulat-

ing the pulse frequency to control the power delivered

to the output. The devices switch only as needed to

service the load, resulting in the highest possible effi-

ciency at light loads. Output current capability in PFM

mode is 150mA (max). The output voltage is typically

1% higher than in PWM mode.

During PFM operation, the error comparator detects the

output voltage falling out of regulation and sets a flip-

flop, which turns on the N-channel MOSFET switch

(Figure 4). When the inductor current ramps to the PFM

mode current limit (800mA typical) and stores a fixed

amount of energy, the current-sense comparator resets

a flip-flop. The flip-flop turns off the N-channel switch

and turns on the P-channel synchronous rectifier. A

second flip-flop, previously reset by the switch’s “on” sig-

nal, inhibits the error comparator from initiating another

cycle until the energy stored in the inductor is transferred

to the output filter capacitor and the synchronous rectifier

current has ramped down to 80mA. This forces operation

with a discontinuous inductor current.

Synchronous Rectifier

The MAX1703 features an internal 140m

Ω, P-channel

synchronous rectifier to enhance efficiency. Synchro-

nous rectification provides a 5% efficiency improve-

ment over similar nonsynchronous boost regulators. In

PWM mode, the synchronous rectifier is turned on dur-

ing the second half of each switching cycle. In low-

power mode, an internal comparator turns on the

synchronous rectifier when the voltage at LX exceeds

the boost regulator output, and then turns it off when

the inductor current drops below 80mA.

Low-Voltage Start-Up Oscillator

The MAX1703 uses a CMOS, low-voltage start-up oscil-

lator for a 1.1V guaranteed minimum start-up input volt-

age at +25°C. On start-up, the low-voltage oscillator

switches the N-channel MOSFET until the output volt-

age reaches 2.15V. Above this level, the normal boost-

converter feedback and control circuitry take over.

Once the device is in regulation, it can operate down to

a 0.7V input, since internal power for the IC is boot-

strapped from the output via the OUT pin. Do not apply

full load until the output exceeds 2.3V (max).

1-Cell to 3-Cell, High-Power (1.5A),

Low-Noise, Step-Up DC-DC Converter

10

______________________________________________________________________________________

2.7A TYP

CURRENT

LIMIT

OSC

FB

R

Q

S

REF

LXP

LXN

POUT

PGND

P

N

Figure 3. Simplified PWM Controller Block Diagram

ERROR

COMPARATOR

FB

REF

800mA TYP

CURRENT

LIMIT

LOGIC HIGH

POUT

P

N

PGND

S

S

Q

R

D

Q

R

LXP

LXN

Figure 4. Controller Block Diagram in Low-Power PFM Mode