Detailed description, Pin description – Rainbow Electronics MAX863 User Manual

Page 5

MAX863

Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

_______________________________________________________________________________________

_______________Detailed Description

The MAX863 dual, bi-CMOS, step-up, switch-mode
power-supply controller provides preset 3.3V, 5V, or
adjustable outputs. Its pulse-frequency-modulated
(PFM) control scheme combines the advantages of low
supply current at light loads and high efficiency with
heavy loads. These attributes make the MAX863 ideal
for use in portable battery-powered systems where
small size and low cost are extremely important, and
where low quiescent current and high efficiency are
needed to maximize operational battery life. Use of
external current-sense resistors and MOSFETs allows
the designer to tailor the output current and voltage
capability for a diverse range of applications.

PFM Control Scheme

Each DC-DC controller in the MAX863 uses a one-shot-
sequenced, current-limited PFM design, as shown in
Figure 1. Referring to the Typical Operating Circuit
(Figure 2) and the switching waveforms (Figures 3a–3f),
the circuit works as follows. Output voltage is sensed
by the error comparator using either an internal voltage
divider connected to SENSE1 or an external voltage
divider connected to FB1. When the output voltage
drops, the error comparator sets an internal flip-flop.
The flip-flop turns on an external MOSFET, which allows
inductor current to ramp-up, storing energy in a mag-
netic field.

______________________________________________________________Pin Description

PIN

Feedback Input for DC-DC Controller 1 in Fixed-Output Mode

SENSE1

FUNCTION

NAME

IC Power-Supply Input

Bootstrap Low-Voltage-Oscillator Enable Input. BOOT is an active-high, logic-level input. It enables the
low-voltage oscillator to allow start-up from input voltages down to 1.5V while in a bootstrapped circuit
configuration. Connect BOOT to GND when in a non-bootstrapped configuration. If BOOT is high, V

must be connected to OUT1.

BOOT

Adjustable Feedback and Preset Output Voltage Selection Input for DC-DC Controller 1. Connect to V

for 3.3V preset output or to GND for 5V output. Connect a resistor voltage divider to adjust the output volt-
age. See the section Set the Output Voltage.

FB1

Gate-Drive Output of DC-DC Controller 1. Drives an external N-channel power MOSFET.

EXT1

High-Current Ground Return for Internal MOSFET Drivers

PGND

Analog Ground for Internal Reference, Feedback, and Control Circuits

GND

Input to the Current-Sense Comparator of DC-DC Controller 1

CS1

Input to the Current-Sense Amplifier of DC-DC Controller 2

CS2

Adjustable Feedback Input for DC-DC Controller 2. Connect a resistor voltage divider to adjust the output
voltage. See the section Set the Output Voltage.

FB2

Active-Low Shutdown Input for DC-DC Controller 1. Connect to V

for normal operation.

SHDN1

Low-Battery Comparator Input. When the voltage on LBI drops below 1.25V, LBO sinks current. If unused,
connect to GND.

LBI

Reference Bypass Input. Connect a 0.1µF ceramic capacitor from REF to GND.

REF

Active-Low Shutdown Input for DC-DC Controller 2. Connect to V

for normal operation.

SHDN2

Low-Battery Output. An open-drain N-channel MOSFET output. Sinks current when the voltage on LBI
drops below 1.25V. If unused, connect to GND.

LBO

Gate-Drive Output of DC-DC Controller 2. Drives an external N-channel power MOSFET.

EXT2