Step-Up/Down DC-DC Converter

in QSOP Package

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7

_______________Detailed Description

The MAX1672 integrates a step-up, switch-mode DC-

DC converter with a linear regulator to provide step-

up/down voltage conversion. The step-up converter

contains an N-channel power MOSFET switch, while

the linear regulator contains a P-channel MOSFET pass

element (Figure 1). The step-up converter and the lin-

ear regulator share the same precision voltage refer-

ence. The MAX1672’s input range is from +1.8V to

+11V, and the regulated output is internally preset to

+3.3V or +5V, or can be adjusted with two external

resistors. Boost efficiency typically exceeds 80% over a

2mA to 200mA load range. The device is bootstrapped

with chip power derived from the stepped-up voltage

output at PS. The MAX1672 typically starts up with a

0.9V input.

The MAX1672’s step-up/linear-regulator configuration

permits the use of a physically smaller inductor than

competing SEPIC and flyback configurations because

those of SEPIC and flyback converters. Also, high-fre-

quency switching and selectable peak inductor current

limit allow for low inductor value (10µH) and low current

saturation rating, respectively, further reducing the

inductor’s physical dimensions.

The MAX1672 maximizes efficiency in both step-up and

ear regulator behaves as a 1.2

Ω (at 5V output) PFET

switch. This provides optimum efficiency (typically

85%).

linear regulator are active. The step-up regulator is

automatically enabled to maintain headroom across the

linear regulator (typically 1V above the 5V output). In

this case, boost ripple is rejected by the linear regula-

tor, and OUT remains in regulation with no dropout.

The mode of operation is automatically controlled on-

down, and normal step-down operation are stable, but

can be seen as small variations in the output DC level

and output ripple.

Step-Up Switch-Mode Converter

A pulse-frequency-modulation (PFM) control scheme,

with a constant 1µs off-time and variable on-time, con-

trols the N-channel MOSFET switch. A pulse is initiated

whenever OUT falls out of regulation. The N-channel

switch then turns off when the inductor current reaches

the peak current limit or after the 4µs maximum on-time,

whichever occurs first. This control architecture pro-

vides high-efficiency, discontinuous inductor current

under light loads as well as continuous inductor current

under heavy loads. The switching frequency and output

ripple are a function of load current and input voltage.

Linear Regulator

The low-dropout linear regulator consists of a refer-

ence, an error amplifier, and a P-channel MOSFET. The

reference is connected to the error amplifier input. The

error amplifier compares this reference with the select-

ed feedback voltage and amplifies the difference. The

difference is conditioned and applied to the P-channel

pass transistor’s gate.

ILIM

The current-limit-select input, ILIM, selects between the

two peak inductor current limits: 0.8A (ILIM = PS) and

0.5A (ILIM = GND). If the application requires low out-

put current (see Typical Operating Characteristics),

select 0.5A. The lower peak current limit allows for a

smaller, lower-cost inductor, and reduced output ripple.

On/Off Control

The MAX1672 is turned on or off by logic inputs ONA

and ONB (Table 1). When ONA = 1 or ONB = 0, the

device is on. When ONA = 0 and ONB = 1, the device

shuts down (see the Applications Information section).

For normal (on) operation, connect ONA to PS and

ONB to GND. Shutdown mode turns off the MAX1672

completely, disconnecting the input from the output

and actively pulling OUT to GND.

ONB

MAX1672

0

0

On

0

1

Off

ONA

1

0

On

1

1

On

Table 1. On/Off Logic Control