1- to 3-Cell, High-Current, Low-Noise,

Step-Up DC-DC Converters with Linear Regulator

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13

The linear regulator in the MAX1705/MAX1706 features

a 0.5

Ω, P-channel MOSFET pass transistor. This pro-

vides several advantages, including longer battery life,

over similar designs using a PNP pass transistor. The

P-channel MOSFET requires no base-drive current,

which reduces quiescent current considerably. PNP-

based regulators tend to waste base-drive current in

dropout when the pass transistor saturates. The

MAX1705/MAX1706 eliminate this problem.

The linear-regulator error amplifier compares the output

feedback sensed at the FBLDO input against the inter-

nal 1.250V reference, and amplifies the difference

(Figure 1). The MOSFET driver reads the error signal

and applies the appropriate drive to the P-channel

pass transistor. If the feedback signal is lower than the

reference, the pass-transistor gate is pulled lower,

allowing more current to pass to the output, thereby

increasing the output voltage. If the feedback voltage is

too high, the pass-transistor gate is pulled up, allowing

less current to pass to the output. Additional blocks

include a current-limiting block and a thermal-overload

protection block.

Low-Voltage Start-Up Oscillator

The MAX1705/MAX1706 use a CMOS, low-voltage

start-up oscillator for a 1.1V guaranteed minimum start-

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

oscillator switches the N-channel MOSFET until the out-

put voltage reaches 2.15V. Above this level, the normal

step-up 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

bootstrapped from the output using the OUT pin.

To reduce current loading during step-up, the linear

regulator is kept off until the start-up converter goes

into regulation. Minimum start-up voltage is influenced

by load and temperature (see the

Typical Operating

Characteristics). To allow proper start-up, do not apply

a full load at POUT until after the device has exited

start-up mode and entered normal operation.

Shutdown

The MAX1705/MAX1706 feature a shutdown mode that

reduces quiescent current to less than 1µA, preserving

battery life when the system is not in use. During shut-

down, the reference, the low-battery comparator, and

all feedback and control circuitry are off. The step-up

converter’s output drops to one Schottky diode drop

below the input, but the linear regulator output is

turned off.

Entry into shutdown mode is controlled by logic input

pins ONA and ONB (Table 2). Both inputs have trip

Tracking

Connecting TRACK to the step-up converter output

implements a tracking mode that sets the step-up

converter output to 300mV above the linear-regulator

output, improving efficiency. In track mode, feedback

for the step-up converter is derived from the OUT pin.

When TRACK is low, the step-up converter and linear

regulator are separately controlled by their respective

feedback inputs, FB and FBLDO. TRACK is a logic

wired or switched with a slew rate exceeding 1V/µs.

properly.

On power-up with TRACK = OUT, the step-up convert-

er initially uses the FB input to regulate its output. After

the step-up converter goes into regulation for the first

time, the linear regulator turns on. When the linear regu-

lator reaches 2.3V, track mode is enabled and the step-

up converter is regulated to 300mV above the linear-

regulator output.

Low-Battery Comparator

The internal low-battery comparator has uncommitted

inputs and an open-drain output capable of sinking

1mA. To use it as a low-battery-detection comparator,

connect the LBN input to the reference, and connect

the LBP input to an external resistor divider between

the positive battery terminal and GND (Figure 2). The

resistor values are then as follows:

LBP is less than 50nA, R6 can be a large value (such

as 270k

Ω or less) without sacrificing accuracy.

Connect the resistor voltage divider as close to the IC

as possible, within 0.2in. (5mm) of the LBP pin. The

inputs have a 0.5V to 1.5V common-mode input range,

and a 16mV input-referred hysteresis.

R

R

V

V

IN TH

LBN

5

6

,

=









- 1

ONA

ONB

MAX1705/MAX1706

0

0

On

0

1

Off

1

0

On

1

1

On

Table 2. On/Off Logic Control