Max6948b, Detailed description, Functional block diagram – Rainbow Electronics MAX6948B User Manual

Page 9

High-Efficiency PWM LED Driver with Boost

Converter and Five Constant-Current GPIO Ports

MAX6948B

Detailed Description

The MAX6948B general-purpose input/output (GPIO)
peripheral with integrated boost converter provides a
boost converter capable of driving 6 WLEDs and five I/O
ports capable of driving LEDs powered from an alternate
power supply such as the Li+ battery. The integrated
2MHz boost converter minimizes the size and cost of
external components and supplies 30mA of load current
at up to 28V. The feedback input to the error amplifier
has a typical set point of 0.1V to minimize power dis-
sipation. External compensation keeps the converter
stable under all load conditions from 5V up to 28V. The
MAX6948B includes overvoltage and open-circuit detec-
tion to prevent damage to the IC.
An I

C-programmable 10-bit PWM signal enables 1024

levels of WLED intensity. During PWM off-time, the
internal switch at the LEDSW pin disconnects the series
WLEDs. This limits the PWM off-time leakage current to
a minimum, limited only by the PWM switch internal to
the MAX6948B. Consequently, the boost output voltage

remains almost constant during PWM on-/off-time peri-
ods. This new approach provides advantages of minimal
WLED color change for sharp WLED on and off, and
more power efficiency due to minimal leakage.
The five GPIO ports function as logic inputs, open-
drain logic outputs, or constant-current sinks in any
combination. Ports withstand 5.5V independent of the
MAX6948B’s supply voltage. Two of the ports drive
additional LEDs up to 30mA, while the other three ports
drive LEDs at up to 10mA/port. The MAX6948B features
shutdown and standby modes for low-power dissipa-
tion. The constant-current drivers contain programmable
PWM outputs and allow staggering to reduce the input
peak current requirements. The I/O ports also feature
ramp-up and ramp-down controls.
The MAX6948B features a single input to select from four
I

C slave addresses. Programming and functionality for

the five GPIO ports is identical to the MAX6946/MAX6947
I/O expanders.

Functional Block Diagram

SDA

SCL

ADO

THERMAL

SHUTDOWN

125kHz, 31.25kHz

OSCILLATOR

BANDGAP

REFERENCE

CURRENT

DAC

n-CHANNEL
MOSFET

LX
LX

PGND
PGND

COMP

LEDSW

POR

INTERFACE

CONTROL

REGISTERS

PWM AND

GPIO LOGIC

LED ENABLE

GPIO ENABLE

GPIO INPUT

PORT GPIO

AND

CONSTANT-

CURRENT

LED DRIVE

OVP

UVLO

RST

OUT

V+

P1
P2
P3
P4

MAX6948B

PWM AND

GATE DRIVE