11
FN9198.4
April 17, 2009
Applications Information
The ISL97650 provides a complete power solution for TFT
LCD applications. The system consists of one boost
converter to generate 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 to use 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.
The boost converter 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 2.6Apeak.
This restricts the maximum output current (average) based
on Equation 3:
Where IL 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, fs and I
OMAX
):
The minimum duty cycle of the ISL97650 is 25%. When the
operating duty cycle is lower than the minimum duty cycle,
the part will not switch in some cycles randomly, which will
cause some LX pulses to be skipped. In this case, LX pulses
are not consistent any more, but the output voltage (A
VDD
) is
still regulated by the ratio of R3 and R5. This relationship is
given by Equation 2. Because some LX pulses are skipped,
the ripple current in the inductor will become bigger. Under
the worst case, the ripple current will be from 0 to the
threshold of the current limit. In turn, the bigger ripple current
will increase the output voltage ripple. Hence, it will need
more output capacitors to keep the output ripple at the same
level. When the input voltage equals, or is larger than, the
output voltage, the boost converter will stop switching. The
boost converter is not regulated any more, but the part will
still be on and other channels are still regulated. The typical
waveforms of pulse-skipping mode are shown in the section
“Typical Performance Curves” on page 5.
Boost Converter Input Capacitor
An input capacitor is used to suppress the voltage ripple
injected into the boost converter. The ceramic capacitor with
capacitance larger than 10µF is recommended. The voltage
rating of input capacitor should be larger than the maximum
input voltage. Some capacitors are recommended in Table 2
for input capacitor.
V
boost
V
IN
------------------
1
1D–
-------------
=
(EQ. 1)
A
VDD
R
3
R
5
+
R
5
---------------------
V
FBB
=
(EQ. 2)
I
OMAX
I
LMT
I
L
2
--------
–
V
IN
V
O
---------
=
(EQ. 3)
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
V
IN
(V)
V
O
(V)
L
(µH)
f
s
(MHz)
I
OMAX
(mA)
5 9 6.8 1.2 1138
5 12 6.8 1.2 777
4 15 6.8 1.2 560
12 15 6.8 1.2 1345
12 18 6.8 1.2 998
TABLE 2. BOOST CONVERTER INPUT CAPACITOR
RECOMMENDATION
CAPACITOR SIZE VENDOR PART NUMBER
10µF/25V 1210 TDK C3225X7R1E106M
10µF/25V 1210 Murata GRM32DR61E106K
I
L
V
IN
L
---------
D
f
S
-----
=
(EQ. 4)
ISL97650
