ML4836
FUNCTIONAL DESCRIPTION
(Continued)
The ML4836 implements a triple frequency operation
scheme: programmable three-frequency sequence for pre-
heat, ignition, and dimming, that extends lamp life,
simplifies lamp network design, and starts lamps at any
dimming level without flashing. This addresses the need
for a high-Q network for starting sequence and low-Q
network for operation, minimizing parasitic losses and
improving overall power efficiency. The values for the
pre-heat, start, operation, and restart can be programmed
or selected (Figure 2).
TRANSCONDUCTANCE AMPLIFIERS
The Lamp voltage feedback amplifier is implemented as
an operational transconductance amplifier. It is designed
to have low small signal forward transconductance such
that a large value of load resistor (R1) and a low value
ceramic capacitor (<1µF) can be used for AC coupling
(C1) in the frequency compensation network. The
compensation network shown in Figure 5 will introduce a
zero and a pole at:
f
Z
=
1
2
p
R
1
C
1
f
P
=
1
2
p
R
1
C
2
(2)
Figure 3 shows the output configuration for the operational
transconductance amplifiers.
A DC path to ground or V
CC
at the output of the
transconductance amplifiers will introduce an offset error.
PREHEAT
SET TIME VALUES
FOR PREHEAT,
START AND OPERATION,
AND RESTART
f1
ML4836
HIGH Q
LOW Q
f2
START
f3
OPERATION
Figure 2. Three Frequency Design Model
CURRENT
MIRROR
IN
gmV
IN
2
io = gmV
IN
OUT
IQ +
IQ –
gmV
IN
2
IN
OUT
CURRENT
MIRROR
Figure 3. Output Configuration
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
