el7585a Intersil Corporation, el7585a Datasheet - Page 9

• Manufacturer Part Number: el7585a
• Description: Tft-lcd Power Supply
• Manufacturer: Intersil Corporation

Boost Converter
The main boost converter is a current mode PWM converter at
a fixed frequency of 1MHz which enables the use of low profile
inductors and multilayer ceramic capacitors. This results in a
compact, low cost power system for LCD panel design.
The EL7585A is 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 the switching MOSFET.
Figure 19 shows the block diagram of the boost regulator. 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.
A
--------------- -
V
VDD
IN
=
------------ -
1 D
–
1
FBB
9
Ifb
Iref
FIGURE 19. BLOCK DIAGRAM OF THE BOOST REGULATOR
COMPENSATION
AMPLIFIER
AMPLIFIER
CURRENT
GENERATOR
REFERENCE
GM
SLOPE
CINT
EL7585A
CLOCK
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 current through the MOSFET is limited to 3.5A peak.
This restricts the maximum output current based on the
following equation:
I
Where ∆IL is peak to peak inductor ripple current, and is set by:
where f
A
OMAX
AMPLIFIER
∆I
LOGIC
VOLTAGE
PWM
VDD
L
=
=
=
V
---------
S
L
IN
R
-------------------- -


is the switching frequency.
I
1
LMT
R
×
+
1
---- -
f
D
R
S
SHUTDOWN
& START-UP
BUFFER
CONTROL
2
–
×
∆I
--------
2
V
L
REF


Iref
Ifb
×
V
---------
V
IN
O
PGND
LX
March 9, 2006
FN7523.3