Micrel, Inc. MIC4827
July 2009 7
M9999-070709-A
(408) 955-1690
Block Diagram
Q
1
Q
2
/Q
3
/Q
4
EL
Oscillator
GND
V
REF
4
VB
EL LAMP
7
VA
8
CS
6
SW
C
OUT
D1
L1
220µH
5
V
DD
1
RSW
R
SW
C
IN
2
REL
R
EL
3
Switch
Oscillator
V
IN
Figure 1. MIC4827 Block Diagram
Functional Description
Overview
The MIC4827 is a high-voltage EL driver with an AC
output voltage of 180V peak-to-peak capable of driving
EL lamps up to 6in
2
. Input supply current for the
MIC4827 is typically 21µA with a typical shutdown
current of 10nA. The high voltage EL driver has two
internal oscillators to control the switching MOSFET and
the H-bridge driver. Both of the internal oscillators’
frequencies can be individually programmed through the
external resistors to maximize the efficiency and the
brightness of the lamps.
Regulation
Referring to Figure 1, initially power is applied to V
DD
.
The internal feedback voltage is less than the reference
voltage causing the internal comparator to go low which
enables the switching MOSFET’s oscillator. When the
switching MOSFET turns on, current flows through the
inductor and into the switch. The switching MOSFET will
typically turn on for 90% of the switching frequency.
During the on-time, energy is stored in the inductor.
When the switching MOSFET turns off, current flowing
into the inductor forces the voltage across the inductor to
reverse polarity. The voltage across the inductor rises
until the external diode conducts and clamps the voltage
at V
OUT
+ V
D1
. The energy in the inductor is then
discharged into the C
OUT
capacitor. The internal
comparator continues to turn the switching MOSFET on
and off until the internal feedback voltage is above the
reference voltage. Once the internal feedback voltage is
above the reference voltage, the internal comparator
turns off the switching MOSFET’s oscillator.
When the EL oscillator is enabled, V
A
and V
B
switch in
opposite states to achieve a 180V peak-to-peak AC
output signal. The external resistor that connects to the
REL pin determines the EL frequency.
