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ISL97650 Datasheet(PDF) 11 Page - Renesas Technology Corp

No. de pieza ISL97650
Descripción Electrónicos  4-Channel Integrated LCD Supply
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Fabricante Electrónico  RENESAS [Renesas Technology Corp]
Página de inicio  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

ISL97650 Datasheet(HTML) 11 Page - Renesas Technology Corp

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ISL97650
FN9198 Rev.4.00
Page 11 of 20
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 AVDD voltage for column drivers, one buck
converter to provide voltage to logic circuit in the LCD panel,
one integrated VON charge pump and one VOFF linear-
regulator controller to provide the voltage to row drivers. This
part also integrates VON-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 fs is the switching frequency(1.2MHz).
Table 1 gives typical values (margins are considered 10%, 3%,
20%, 10% and 15% on VIN, VO, L, fs and IOMAX):
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 (AVDD) 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.
Boost Inductor
The boost inductor is a critical part which influences the output
voltage ripple, transient response, and efficiency. Values of
3.3µH to 10µH are to match the internal slope compensation.
Vboost
VIN
------------------
1
1D
-------------
=
(EQ. 1)
AVDD
R3 R5
+
R5
--------------------- VFBB
=
(EQ. 2)
IOMAX
ILMT
I
L
2
--------

 VIN
VO
---------
=
(EQ. 3)
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
VIN
(V)
VO
(V)
L
(µH)
fs
(MHz)
IOMAX
(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
VIN
L
---------
D
fS
-----
=
(EQ. 4)


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