11
FN7493.3
April 24, 2009
Applications Information
The ISL97651 provides a complete power solution for TFT
LCD applications. The system consists of one boost
converter to generate the A
VDD
voltage for column drivers,
one buck converter to provide voltage to logic circuit in the
LCD panel, one integrated V
ON
charge pump and one V
OFF
linear-regulator controller to provide the voltage to row
drivers. This part also integrates V
ON
-slice circuit which can
help to optimize the picture quality. With the high output
current capability, this part is ideal for big screen LCD TV
and monitor panel application.
The integrated boost converter and buck converter operate
at 1.2MHz which can allow the use of multilayer ceramic
capacitors and low profile inductor which result in low cost,
compact and reliable system. The logic output voltage is
independently enabled to give flexibility to the system
designers.
Boost Converter
The boost converter is a current mode PWM converter
operating at a fixed frequency of 1.2MHz. It can operate in
both discontinuous conduction mode (DCM) at light load and
continuous mode (CCM). 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 the switching MOSFET
Figure 11 shows the functional 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.
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 60k is recommended.
The boost converter output voltage is determined by
Equation 2:
The current through the MOSFET is limited to a minimum of
4.4A
PEAK
(maximum values can be up to 6.3A
PEAK
.
This restricts the maximum output current (average) based
on Equation 3:
Where I
L
is peak to peak inductor ripple current, and is set
by Equation 4:
where f
S
is the switching frequency (1.2MHz).
Table 1 gives typical values (margins are considered 10%,
3%, 20%, 10% and 15% on V
IN
, V
O
, L, f
S
and I
OMAX
:
Boost Converter Input Capacitor
An input capacitor is used to suppress the voltage ripple
injected into the boost converter. A ceramic capacitor with
capacitance larger than 10µF is recommended. The voltage
rating of input capacitor should be larger than the maximum
input voltage. Examples of recommended capacitors are
given in Table 2 below.
Boost Inductor
The boost inductor is a critical component which influences
the output voltage ripple, transient response, and efficiency.
Values of 3.3µH to 10µH are to match the internal slope
compensation. The inductor must be able to handle without
saturating the following average and peak current:
V
BOOST
V
IN
------------------------
1
1D–
-------------
=
(EQ. 1)
V
BOOST
R
3
R
5
+
R
5
--------------------------
V
REF
=
(EQ. 2)
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
V
IN
(V) V
O
(V) L (µH) F
S
(MHz) I
OMAX
(mA)
4 9 6.8 1.2 1661
4 12 6.8 1.2 1173
4 15 6.8 1.2 879
5 9 6.8 1.2 2077
5 12 6.8 1.2 1466
5 15 6.8 1.2 1099
TABLE 2. BOOST CONVERTER INPUT CAPACITOR
RECOMMENDATION
CAPACITOR SIZE VENDOR PART NUMBER
10µF/16V 1206 TDK C3216X7R1C106M
10µF/10V 0805 Murata GRM21BR61A106K
22µF/10V 1210 Murata GRB32ER61A226K
I
OMAX
I
LMT
I
L
2
--------
–
V
IN
V
O
---------
=
(EQ. 3)
I
L
V
IN
L
---------
D
f
S
-----
=
(EQ. 4)
I
LAVG
I
O
1D–
-------------
=
I
LPK
I
LAVG
I
L
2
--------
+=
(EQ. 5)
ISL97651
