ISL97642 Intersil Corporation, ISL97642 Datasheet - Page 10

• Manufacturer Part Number: ISL97642
• Description: TFT-LCD DC/DC
• Manufacturer: Intersil Corporation

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Boost Converter
The main boost converter is a current mode PWM converter
operating at a fixed frequency. The 1.2MHz switching
frequency enables the use of low profile inductor and
multilayer ceramic capacitors, which results in a compact,
low cost power system for LCD panel design.
The boost converter can operate in continuous or
discontinuous inductor current mode. The ISL97642 is
designed for continuous current mode, but it can also
operate in discontinuous current mode at light load. In
continuous current mode, current flows continuously in the
inductor during the entire switching cycle in steady state
operation. The voltage conversion ratio in continuous current
mode is given by Equation 1:
Where D is the duty cycle of switching MOSFET.
V
----------------------- -
BOOST
V
IN
=
------------ -
1 D
DRVN
–
C
1
FBB
FBN
INT
BUFFER
SHUTDOWN
V
THERMAL
REF
10
COMPARATOR
AMPLIFIER
0.4V
GENERATOR
REFERENCE
GM
UVLO
COMPARATOR
SS
+
-
UVLO
COMPENSATION
AND START-UP
0.2V
SHUTDOWN
CONTROL
FIGURE 16. BLOCK DIAGRAM
SLOPE
(EQ. 1)
OSCILLATOR
ISL97642
Σ
LIMIT COMPARATOR
COMP
COMPARATOR
CONTROLLER
AMPLIFIER
CURRENT
CURRENT
UVLO
LOGIC
PWM
OSC
Figure 17 shows the block diagram of the boost controller.
It uses a summing amplifier architecture consisting of GM
stages for voltage feedback, current feedback and slope
compensation. A comparator looks at the peak inductor
current cycle by cycle and terminates the PWM cycle if the
current limit is reached.
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. Current drawn
by the resistor network should be limited to maintain the
overall converter efficiency. The maximum value of the
resistor network is limited by the feedback input bias current
and the potential for noise being coupled into the feedback
pin. A resistor network in the order of 60kΩ is recommended.
The boost converter output voltage is determined using
Equation 2:
V
BOOST
V
REF
CURRENT REF
=
R
-------------------- -
1
R
+
1
BUFFER
SS
R
+
2
-
×
V
REF
BUFFER
LX
PGND
DRVP
FBP
June 18, 2007
(EQ. 2)
FN6436.0