1-Cell to 3-Cell, High-Power,

Low-Noise, Step-Up DC-DC Converters

10

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Low-Power PFM Mode

When CLK/SEL is pulled low, the MAX848/MAX849 oper-

ate in low-power, low-supply-current PFM mode. Pulse-

frequency modulation provides the highest efficiency at

light loads. The P-channel rectifier is turned off to reduce

gate-charge losses, and the regulator operates in dis-

continuous mode. The N-channel power MOSFET is kept

on until the inductor current ramps to 30% of the current

limit. The inductor energy is delivered to the output

capacitor when the switch turns off. A new cycle is inhib-

ited until the inductor current crosses zero. Zero current

detection is accomplished by sensing the LX voltage

crossing the output voltage. Figure 3 shows the block

diagram for the PFM controller.

Low-Noise PWM Mode

When CLK/SEL is pulled high, the MAX848/MAX849

operate in high-power, low-noise, current-mode PWM,

switching at the 300kHz nominal internal oscillator fre-

quency. The internal rectifier is active in this mode,

and the regulator operates in continuous mode. The

N-channel power MOSFET turns on until either the output

voltage is in regulation or the inductor current limit is

reached (0.8A for the MAX848 and 1.4A for the

MAX849). The switch turns off for the remainder of the

cycle and the inductor energy is delivered to the output

capacitor. A new cycle is initiated on the next oscillator

cycle. In low-noise applications, the fundamental and the

harmonics generated by the fixed switching frequency

can easily be filtered. Figure 4 shows the block diagram

for the PWM controller.

The MAX848/MAX849 enter synchronized current-mode

applied to CLK/SEL. The internal synchronous rectifier

is active and the switching frequency is synchronized

to the externally applied clock signal. For wireless

applications, this ensures that the harmonics of the

switching frequencies are predictable and can be kept

outside the IF band(s). High-frequency operation per-

mits low-magnitude output ripple voltage.

The MAX848/MAX849 are capable of providing a stable

output even with a rapidly pulsing load (GSM, DECT),

such as from a transmitter power amplifier in digital cord-

less phones (see

Typical Operating Characteristics).

In PWM mode, the use of the synchronous rectifier

ensures constant-frequency operation, regardless of

the load current.

Setting the Output Voltage Externally

The MAX848/MAX849 feature Dual Mode operation.

The output voltage is preset to 3.3V (FB = 0V), or it can

be adjusted from 2.7V to 5.5V with external resistors

R1, R2, and R3, as shown in Figure 5. To set the output

voltage externally, select resistor R3 in the 10k

Ω to

100k

Ω range. The values for R1 and R2 are given by:

MAX849

C5

0.1

µF

C2

0.1

µF

C3

0.22

µF

C1

22

µF

OUT

GND

POK

ON1

ON2

CLK/SEL

REF

PGND

FB

POKIN

LX

POUT

C4

2 x 100

µF

L1

10

µH

D1

MBR0520L

3.3V @

200mA

R3

100k

10

Ω

HIGH-CURRENT PATH.

*

Figure 2. 3.3V Preset Output

Table 2. Selecting Operating Mode

CLK/SEL

MODE

0

PFM

1

PWM

External clock

(200kHz ~ 400kHz)

Synchronized PWM

R

S

Q

R

D

Q

PFM-MODE

CURRENT-

LIMIT LEVEL

REF

FEEDBACK

POUT

LX

PGND

LOGIC HIGH

N

CURRENT

SENSE

Figure 3. Controller Block Diagram in PFM Mode