Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

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11

During PFM operation, the error comparator detects

when the output voltage is out of regulation and sets a

flip-flop, turning on the N-channel MOSFET switch

(Figure 5). When the inductor current ramps to the PFM

mode current limit (350mA), the current-sense compara-

tor resets a flip-flop. The flip-flop turns off the N-channel

switch and turns on the P-channel synchronous rectifier.

The energy stored in the inductor is transferred to the

output through the P-channel switch. A second flip-flop,

previously reset by the switch’s “on” signal, inhibits the

next cycle until the inductor current is depleted and the

output is out of regulation. This forces operation with

discontinuous inductor current in PFM mode.

Startup Oscillator

The MBC employs a low-voltage startup oscillator to

ensure a 1.1V (0.9V typical) startup voltage. On start-

up, if the output voltage is less than 2.25V, the P-chan-

nel switch stays off and the N-channel pulses at a 25%

duty cycle. When the output voltage exceeds 2.25V,

the normal PWM or PFM control circuitry takes over.

Once the MBC is in regulation, it can operate with

inputs down to 0.7V since the internal power for the IC

is taken from OUT. The MBC cannot supply full output

current until OUT reaches 2.5V.

Synchronous Rectifier

The MAX1677 MBC features an internal 1

Ω P-channel

synchronous rectifier. Synchronous rectification typical-

ly improves efficiency by 5% or more over similar non-

synchronous step-up designs. In PWM mode, the

synchronous rectifier turns on during the second half of

each cycle. In PFM mode, an internal comparator turns

on the synchronous rectifier when the voltage at LX

exceeds the MBC output, and then turns it off when the

inductor current drops below 90mA (typ).

The on-chip synchronous rectifier allows the external

Schottky diode to be omitted in designs that operate

from inputs exceeding 1.4V. In circuits operating below

1.4V (1-cell inputs, for example), connecting a Schottky

diode in parallel with the internal synchronous rectifier

(from LX to POUT) provides the lowest startup voltage.

LCD Boost Converter (LCD)

The LCD converter can be configured for a positive or

negative output by setting the LCDPOL pin and using

the appropriate circuit (Figures 2 and 3, and Table 3).

A combination of peak current limiting and a pair of

one-shot timers control LCD switching. During the on-

cycle the internal N-channel DMOS switch turns on,

and inductor current ramps up until either the switch

peak current limit is reached or the 5.2µs maximum on-

time expires (typically at low input voltages). After the

on-cycle terminates, the switch turns off and the output

capacitor charges. The switch remains off until the error

comparator initiates another cycle.

The LCDLX current limit is set by LCDPOL, as outlined

in Table 3. The lower, 225mA peak current setting

allows tiny low-current “chip” inductors to be used

when powering smaller (less than 15 square inches)

liquid crystal panels. Use the following equation to

determine which LCDLX current-limit setting is

required.

350mA or 225mA as set by LCDPOL. The 0.7 term is a

correction factor to conservatively account for typical

switch, inductor, and diode losses.

The LCD boost is enabled when both ON and LCDON

are high, and the MBC output voltage is within 90% of

its set value. A soft-start startup mode with increased

CLK/SEL

0

1

PWM

Low-Power PFM

MBC MODE

FEATURES

Lowest Supply Current

High Output Current,

Fixed-Frequency Ripple

Ext Clock

(200Hz to

400kHz)

Synchronized

PWM

High Output Current,

Synchronized Ripple

Frequency

Table 2. Selecting MBC Operating Mode

R

S

Q

R

D

Q

Q

CURRENT

LIMIT LEVEL

POUT

LX

P

N

PGND

LOGIC HIGH

Figure 5. Controller Block Diagram in PFM Mode