MIC4833 Datasheet, PDF (6/12 Pages) Micrel Semiconductor – Low Noise Dual 220 VPP EL Driver With Output Voltage Slew Rate Control

Micrel
MIC4833
Functional Description
Overview
The MIC4833 is a high-voltage dual output EL driver
with a peak-to-peak AC output voltage of 220V
capable of driving two 4 in² EL panels. The MIC4833
drives EL panels by converting a low DC input
voltage to a DC high output voltage using the boost
regulator circuit and then alternating the high DC
voltage across the EL panel using an H-Bridge.
Input supply current for the MIC4833 is typically
152µA. The high voltage EL driver has two internal
oscillators to control the boost switching frequency
and the H-bridge driver frequency. Both of the
internal oscillators’ frequencies can be individually
programmed through external resistors to maximize
efficiency and brightness of the EL panel. The
MIC4833 can be dimmed using a PWM signal
applied to the REL pin with an external capacitor. An
external resistor can be used to adjust the internal
wave shaping circuit to reduce audible noise.
Regulation
Referring to Figure 1, power is initially applied to
V_DD. When the internal feedback voltage is less than
the reference voltage, the internal comparator
enables switching in the boost circuit. When the
boost regulator is switching, current flows through
the inductor into the switch. The switching MOSFET
will typically turn on for 90% of the switching period.
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 disables
switching. The control circuit will continue to turn the
MOSFET’s on and off to maintain a constant DC
voltage at the CS pin.
When the MIC4833 EL Driver is enabled, ELA and
ELB will switch in opposite states with COM to
achieve a 220V peak-to-peak AC output signal
needed to drive the two EL panels.
Switching Frequency
The switching frequency of the converter is
controlled via an external resistor (R_SW) between
October 2008
RSW and VDD. The switching frequency increases
as the resistor value decreases. In general, the
lower the switching frequency, the greater the input
current is drawn to deliver more power to the output.
Lowering the switching frequency can be used to
drive larger panels. However, the switching
frequency should not be so low as to allow the
voltage at the switch node or the CS pin to exceed
the absolute maximum voltage of those pins. For
resistor value selections, see the “Typical
Characteristics: Switching Frequency vs. SW
Resistor” graph on Page 4 or use the equation
below. The switching frequency range is 35kHz to
350kHz, with an accuracy of ±20%.
f_SW
(kHz)
=
46
R_SW (MΩ)
EL Frequency
The EL panel frequency is controlled via an external
resistor (R_EL) connected between REL and VDD.
The panel frequency increases as the resistor value
decreases. In general, as the EL panel frequency
increases, the amount of current drawn from the
battery will increase. The EL panel brightness is
dependent upon its frequency. For resistor value
selections, see the “Typical Characteristics: EL
Frequency vs. EL Resistor” graph on Page 4 or use
the equation below. The EL panel frequency range
is 100Hz to 1500Hz, with an accuracy of ±20%.
fEL
(Hz)
=
425
REL (MΩ)
Enable Function
There are a few different ways to enable and disable
the MIC4833. The boost regulator may be disabled
by pulling the R_SW resistor to ground. This turns off
both the EL panels by cutting power to the device
completely. The EL panels can also be turned off by
pulling the R_EL resistor to ground. Although this turns
off the H-Bridge and the EL panels, the MIC4833
boost regulator will continue regulate. For individual
panel control, the ENA and ENB pins can be used to
enable ELA and ELB, respectively. Pulling ENA or
ENB high or low will turn ELA and ELB panels on or
off.
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M9999-100308
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