MIC4827
Micrel
Block Diagram
L1
220µH
1
V
IN
V
DD
D1
C
IN
R
SW
2
5
C
OUT
SW
RSW
Switch
Oscillator
Q
1
R
EL
Q
2
EL
Oscillator
/Q
3
7
3
8
6
CS
VA
EL LAMP
V
REF
VB
/Q
4
REL
4
GND
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 oscil-
lators’ 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 oscil-
lator.
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.
V
A
(50V/div)
V
A
— V
B
(50V/div)
V
B
(50V/div)
V
IN
= 3.0V
L = 220µH
C
OUT
= 0.033µF
Lamp = 2in
2
R
SW
= 442k
R
EL
= 2M
TIME (2ms/div)
Figure 2. 108Hz Typical Output Waveform
Switching Frequency
The switching frequency of the converter is controlled via an
external resistor between RSW pin and VDD pin of the
device. The switching frequency increases as the resistor
value decreases. For resistor value selections, see the “Typi-
cal Characteristics: Switching Frequency vs. Switch Resis-
tor” or use the equation below. The switching frequency
range is 8kHz to 200kHz, with an accuracy of
±20%.
f
SW
(kHz)
=
36
R
SW
(M
Ω
)
MIC4827
6
January 2005
MICREL [ MICREL SEMICONDUCTOR ]
