ISL97651 データシートの表示（PDF） - Renesas Electronics
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ISL97651 Datasheet PDF : 20 Pages
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ISL97651
Applications Information
The ISL97651 provides a complete power solution for TFT
LCD applications. The system consists of one boost converter
to generate the 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 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:
V-----B---V-O---I-O-N----S----T- = 1-----–-1----D---
(EQ. 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:
VBOOST = ---R-----3--R--+--5---R----5---- VREF
(EQ. 2)
The current through the MOSFET is limited to a minimum of
4.4APEAK (maximum values can be up to 6.3APEAK.
This restricts the maximum output current (average) based on
Equation 3:
IOMAX
=
IL
M
T
–
---2--I--L-
V-V----I-O-N--
(EQ. 3)
Where IL is peak to peak inductor ripple current, and is set by
Equation 4:
IL = -V---L-I--N-- -fD-S--
(EQ. 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:
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
VIN (V)
4
VO (V)
9
L (µH)
6.8
FS (MHz) IOMAX (mA)
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
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.
TABLE 2. BOOST CONVERTER INPUT CAPACITOR
RECOMMENDATION
CAPACITOR SIZE VENDOR
PART NUMBER
10µF/16V
10µF/10V
22µF/10V
1206
0805
1210
TDK
Murata
Murata
C3216X7R1C106M
GRM21BR61A106K
GRB32ER61A226K
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:
ILAVG = 1----I-–-O----D---
ILPK = ILAVG + ---2--I--L-
(EQ. 5)
FN7493 Rev 3.00
April 24, 2009
Page 11 of 20
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