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C3225X7R1E106M

4-Channel Integrated LCD Supply

Intersil 

ISL97650B

Applications Information

The ISL97650B 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 0.5MHz 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 0.5MHz. 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---o---o----s---t

VIN

=

------1-------

1–D

(EQ. 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 60kΩ is recommended.

The boost converter output voltage is determined by

Equation 2:

AVDD

=

R-----3-----+-----R-----5--

R5

×

VF

B

B

(EQ. 2)

The current through the MOSFET is limited to 3.2Apeak.

This restricts the maximum output current (average) based

on Equation 3:

IOMAX

=

⎛

⎝

IL

M

T

–

-Δ--2--I-L--⎠⎞

× V-----I--N--

VO

(EQ. 3)

Where ΔIL is peak to peak inductor ripple current, and is set

by Equation 4:

ΔIL

=

-V----I--N-- × -D---

L fS

(EQ. 4)

where fs is the switching frequency(0.5MHz).

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

VO

L

fs

IOMAX

(V)

(V)

(µH)

(MHz)

(mA)

5

9

6.8

0.5

1138

5

12

6.8

0.5

777

4

15

6.8

0.5

560

12

15

6.8

0.5

1345

12

18

6.8

0.5

998

The minimum duty cycle of the ISL97650B 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 "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.

TABLE 2. BOOST CONVERTER INPUT CAPACITOR

RECOMMENDATION

CAPACITOR SIZE

VENDOR

PART NUMBER

10µF/25V 1210 TDK

C3225X7R1E106M

10µF/25V 1210 Murata

GRM32DR61E106K

11

FN6748.1

March 22, 2010

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