MAX1642 Datasheet, PDF (6/12 Pages) Maxim Integrated Products – High-Efficiency, Step-Up DC-DC Converters for 1V Inputs

High-Efficiency, Step-Up
DC-DC Converters for 1V Inputs
______________________________________________________________Pin Description
PIN
MAX1642 MAX1643
1
1
2
2
—
3
3
4
4
—
5
5
6
6
7
7
8
8
NAME
BATT
PFI
BATTLO
PFO
SHDN
FB
GND
LX
OUT
FUNCTION
IC Battery-Power Input. Sense input for BATTLO comparator (MAX1643 only).
Power-Fail Input. When the voltage on PFI drops below 614mV, PFO sinks current.
Open-Drain Battery-Low Output. When the voltage at BATT drops below 1V, BATTLO
sinks current.
Open-Drain Power-Fail Output. Sinks current when PFI drops below 614mV.
Active-Low Shutdown Input. Connect to BATT for normal operation.
Feedback Input for adjustable-output operation. Connect FB to an external resistor voltage
divider between OUT and GND. Connect to GND for fixed-output operation.
Ground
N-Channel MOSFET Switch Drain and P-Channel Synchronous-Rectifier Drain
Power Output. Feedback input for fixed 3.3V operation and IC power input. Connect filter
capacitor close to OUT.
_______________Detailed Description
The MAX1642/MAX1643 each consist of an internal 1Ω,
N-channel MOSFET power switch, a built-in synchro-
nous rectifier that acts as the catch diode, an oscillator,
a reference, and PFM control circuitry (Figure 1).
These devices are optimized for applications with
power-management features that operate from one
alkaline cell, such as pagers, remote controls, and bat-
tery-powered instruments. They are designed to meet
the specific demands of the operating states character-
istic of such systems:
1) Primary battery is good and the load is active: In
this state, the system draws tens of milliamperes,
and the MAX1642/MAX1643 typically offer 80% effi-
ciency.
2) Primary battery is good and the load is sleeping: In
this state, the load is drawing hundreds of microam-
peres, and the DC-DC converter IC draws very low
quiescent current. In many applications, the load is
expected to be in this state most of the time.
Operating Principle
The MAX1642/MAX1643 employ a proprietary pulse-
frequency-modulation (PFM) control scheme that com-
bines the ultra-low quiescent current traditional of
pulse-skipping PFM converters with the high-load effi-
ciency of pulse-width-modulation (PWM) converters.
The on-time and minimum off-times are varied as a
function of the input and output voltages:
t_ON =
K
V_BATT
t_OFF(MIN) =
1.2 x K
V_OUT - V_BATT
where K is typically 25V-µs. This enables the
MAX1642/MAX1643 to maintain high efficiency over a
wide range of loads and input/output voltages. The DC-
DC converter is powered from the OUT pin.
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