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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Intersil 

ISL97642

Table 2 gives typical values (margins are considered 10%,

3%, 20%, 10% and 15% on VIN, VO, L, fS and ILMT:

TABLE 2.

VIN (V)

3.3

VO (V)

9

L (µH)

6.8

fS (MHz)

1.2

IOMAX (mA)

890

3.3

12

6.8

1.2

666

3.3

15

6.8

1.2

530

5

9

6.8

1.2

1350

5

12

6.8

1.2

1000

5

15

6.8

1.2

795

Input Capacitor

The input capacitor is used to supply the current to the

converter. It is recommended that CIN be larger than 10μF.

The reflected ripple voltage will be smaller with larger CIN.

The voltage rating of input capacitor should be larger than

the maximum input voltage.

Boost Inductor

The boost inductor is a critical part which influences the

output voltage ripple, transient response, and efficiency.

Value of 3.3μH to 10μH inductor is recommended in

applications to fit the internal slope compensation. The

inductor must be able to handle the following average and

peak current:

ILPK

=

IL

A

V

G

+

-Δ----I-L--

2

ILAVG

=

----I--O-------

1–D

(EQ. 5)

Rectifier Diode

A high-speed diode is desired due to the high switching

frequency. Schottky diodes are recommended because of

their fast recovery time and low forward voltage. The rectifier

diode must meet the output current and peak inductor

current requirements.

Output Capacitor

The output capacitor supplies the load directly and reduces

the ripple voltage at the output. Output ripple voltage

consists of two components: the voltage drop due to the

inductor ripple current flowing through the ESR of output

capacitor, and the charging and discharging of the output

capacitor.

VRIPPLE

=

ILPK

×

ESR

+

-V----O-----–-----V----I--N--

VO

×

------I--O-------

COUT

×

--1--

fS

(EQ. 6)

For low ESR ceramic capacitors, the output ripple is

dominated by the charging and discharging of the output

capacitor. The voltage rating of the output capacitor should

be greater than the maximum output voltage.

NOTE: Capacitors have a voltage coefficient that makes their

effective capacitance drop as the voltage across them increases.

COUT in the Equation 6 assumes the effective value of the capacitor

at a particular voltage and not the manufacturer’s stated value,

measured at zero volts.

Compensation

The ISL97642 incorporates a transconductance amplifier in

its feedback path to allow the user some adjustment on the

transient response and better regulation. The ISL97642

uses current mode control architecture, which has a fast

current sense loop and a slow voltage feedback loop. The

fast current feedback loop does not require any

compensation. The slow voltage loop must be compensated

for stable operation. The compensation network is a series

RC network from COMP pin to ground. The resistor sets the

high frequency integrator gain for fast transient response

and the capacitor sets the integrator zero to ensure loop

stability. For most applications, a 2.2nF capacitor and a

180Ω resistor are inserted in series between COMP pin and

ground. To improve the transient response, either the

resistor value can be increased or the capacitor value can be

reduced, but too high resistor value or too low capacitor

value will reduce loop stability.

Boost Feedback Resistors

As the boost output voltage, VBOOST, is reduced below 12V,

the effective voltage feedback in the IC increases the ratio of

voltage to current feedback at the summing comparator

because R2 decreases relative to R1. To maintain stable

operation over the complete current range of the IC, the

voltage feedback to the FBB pin should be reduced

proportionally (as VBOOST is reduced) by means of a series

resistor-capacitor network (R7 and C7) in parallel with R1,

with a pole frequency (fp) set to approximately 10kHz for C2

(effective) = 10µF and 4kHz for C2 (effective) = 30µF.

R7 = 1 ⁄ 0.1 × R2 – (1 ⁄ R1)^-1

(EQ. 7)

C7 = 1 ⁄ 2 × 3.142 × fp × R7

(EQ. 8)

Linear-Regulator Controllers (VON and VOFF)

The ISL97642 includes 2 independent linear-regulator

controllers, in which there is one positive output voltage

(VON) and one negative voltage (VOFF). The VON and VOFF

linear-regulator controller function diagram, application

circuit and waveforms are shown in Figures 18 and 19

respectively.

12

FN6436.0

June 18, 2007

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