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