el7520a Intersil Corporation, el7520a Datasheet - Page 10

• Manufacturer Part Number: el7520a
• Description: 4-channel Dc/dc Controller
• Manufacturer: Intersil Corporation

Boost Converter
The main boost converter is a current mode PWM controller
operating at a fixed frequency. The 1MHz 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 EL7520 and
EL7520A 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
=
DRVN
------------ -
1 D
C
FBB
FBN
1
–
INT
EN
BUFFER
V
SHUTDOWN
THERMAL
REF
10
COMPARATOR
AMPLIFIER
0.4V
GENERATOR
REFERENCE
GM
UVLO
COMPARATOR
SS
+
-
UVLO
COMPENSATION
0.2V
AMPLIFIER
VOLTAGE
SHUTDOWN
& STARTUP
CONTROL
FIGURE 24. BLOCK DIAGRAM
SLOPE
EL7520, EL7520A
COMPARATOR
OSCILLATOR
UVLO
Σ
V
REF
CURRENT LIMIT
COMPARATOR
COMPARATOR
CONTROLLER
AMPLIFIER
CURRENT
UVLO
LOGIC
PWM
OSC
SS
V
Figure 25 shows the function 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
+
REF
-
BOOST
=
SS
R
-------------------- -
+
GENERATOR
-
1
R
CURRENT
LIMIT REF
+
BUFFER
1
R
2
×
V
REF
BUFFER
BUFFER
DRVB
I
I
DRVP
FBP
DRVL
FBL
SIN
SAD
LX
July 12, 2005
FN7318.0