EL7640 INTERSIL [Intersil Corporation], EL7640 Datasheet - Page 10

• Manufacturer Part Number: EL7640
• Description: TFT-LCD DC/DC with Integrated Amplifiers
• Manufacturer: INTERSIL [Intersil Corporation]

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 EL7640, EL7641,
EL7642 are designed for continuous current mode, but they
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:
Where D is the duty cycle of switching MOSFET.
V
----------------------- -
BOOST
V
IN
=
------------ -
1 D
DRVN
–
C
1
FBB
FBN
INT
BUFFER
V
SHUTDOWN
THERMAL
REF
10
COMPARATOR
AMPLIFIER
0.4V
GENERATOR
REFERENCE
GM
UVLO
COMPARATOR
SS
+
-
UVLO
COMPENSATION
0.2V
AMPLIFIER
VOLTAGE
SHUTDOWN
& STARTUP
EL7640, EL7641, EL7642
CONTROL
FIGURE 17. BLOCK DIAGRAM
SLOPE
OSCILLATOR
Σ
LIMIT COMPARATOR
COMP
COMPARATOR
CONTROLLER
AMPLIFIER
CURRENT
CURRENT
UVLO
LOGIC
PWM
OSC
Figure 18 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 by the
following equation:
V
BOOST
V
REF
CURRENT REF
=
R
-------------------- -
1
R
+
1
BUFFER
SS
R
+
2
-
×
V
REF
BUFFER
LX
PGND
DRVP
FBP
September 26, 2005
FN7415.1