MIC4826

Micrel

MIC4826

8

August 2004

R1

322k

R2

3.32M

Li-Ion Battery

3.0V to 4.2V

C1

0.22

µF/10V

Murata

GRM39X7R 224K10

0.01

µF/100V

GRM40X7R103K

D1

Vishay Telefunken

MCL4148

L1

220

µH

Murata

LQH4C221K04

15

6

7

8

2

3

4

SW

CS

VB

VA

VDD

RSW

REL

GND

MIC4826

C2

10

µF/6.3V

Murata

GRM42-6X5R106K6.3

Application Information

Inductor

In general, smaller value inductors, which can handle more

current, are more suitable to drive larger size lamps. As the

inductor value decreases, the switching frequency (con-

trolled by R

input voltage should be increased. Typically, inductor values

ranging from 220

µH to 560µH can be used. Murata offers the

LQH3C series up to 560

µH and LQH4C series up to 470µH,

with low DC resistance. A 220

µH Murata (LQH4C221K04)

inductor is recommended for driving a lamp size of 3 square

inches. It has a maximum DC resistance of 4.0

Ω.

Diode

The application circuit specifies the 1N4148 or equivalent. It

has a forward current of 150mA and a typical forward voltage

of 950mV. For applications that are not cost driven, a fast

switching diode with lower forward voltage and higher re-

verse voltage can be used to enhance the efficiency, such as

BAV19WS and BAV20WS.

Output Capacitor

Low ESR capacitors should be used at the regulated boost

output (CS pin) of the MIC4826 to minimize the switching

output ripple voltage. Selection of the capacitor value will

depend upon the peak inductor current, inductor size, and the

load. MuRata offers the GRM40 series with up to 0.015

µF at

100V, with a X7R temperature coefficient in 0805 surface-

mount package. Typically, values ranging from 0.01

µF to

0.1

µF at 100V can be used for the regulated boost output

capacitor.

Pre-designed Application Circuits

V

IN

I

IN

V

F

EL

Lamp Size

3.3V

20mA

160V

PP

100Hz

TIME (2ms/div)