I

CONTROL

MIN

¦

gm

R

S

R

R

R

R

Osc/

Ramp

Thermal

shutdown

Light

Load

OVP

Over

Current

Protection

Driver

0.5:

S0

S1

S0

S1

SOFT START

OLED

OLED

5 BIT

CONTROL

1.244V

HWEN/PGEN/GPIO

VIO

SCL

SDA

SET

GND

SUB/FB

MAIN

IN

OVP

SW

500 mV

1.2V

+

-

ILED_MAX =

1.244V

ERROR

AMP

5 BIT

CONTROL

192

GPIO

LM3528

SNVS513B – AUGUST 2008 – REVISED MAY 2013

www.ti.com

BLOCK DIAGRAM

Figure 37. LM3528 Block Diagram

OPERATION DESCRIPTION

The LM3528 Current Mode PWM boost converter operates from a 2.7V to 5.5V input and provides two regulated

outputs for White LED and OLED display biasing. The first output, MAIN, provides a constant current of up to

30mA to bias up to 6 series white LED’s. The second output, SUB/FB, can be configured as a current source for

up to 6 series white LED’s at up to 30mA, or as a feedback voltage pin to regulate a constant output voltage of

up to 21V. When both MAIN and SUB/FB are configured for white LED bias the current for each LED string is

compatible interface and SUB/FB becomes the middle tap of a resistive divider used to regulate the output

voltage of the boost converter.

The core of the LM3528 is a Current Mode Boost converter. Operation is as follows. At the start of each

switching cycle the internal oscillator sets the PWM converter. The converter turns the NMOS switch on, allowing

the inductor current to ramp while the output capacitor supplies power to the white LED’s and/or OLED panel.

The error signal at the output of the error amplifier is compared against the sensed inductor current. When the

sensed inductor current equals the error signal, or when the maximum duty cycle is reached, the NMOS switch

turns off causing the external Schottky diode to pick up the inductor current. This allows the inductor current to

ramp down causing its stored energy to charge the output capacitor and supply power to the load. At the end of

the clock period the PWM controller is again set and the process repeats itself.

12

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