ISL97653AIRZ(2008) Просмотр технического описания (PDF) - Intersil

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ISL97653AIRZ
(Rev.:2008)

5-Channel Integrated LCD Supply

Intersil 

ISL97653A

Application Information

AVDD Boost Converter

The AVDD boost converter features a fully integrated 4.4A

boost FET. The regulator uses a current mode PI control

scheme which provides good line regulation and good

transient response. It can operate in both discontinuous

conduction mode (DCM) at light loads 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 soft-start function is digitally controlled within a

fixed 10ms time frame during which the current limit is

increased in eight linear steps.

The boost converter uses a summing amplifier architecture

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 triggered. Since this comparison is cycle

based, the PWM output will be released after the peak

current goes below the current limit threshold.

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-----4-

R4

×

VF

B

B

(EQ. 2)

where R3 and R4 are in the “” on page 5. Unless otherwise

stated, component variables referred to in equations refer to

the Typical Application Diagram.

The current through the MOSFET is limited to 4.4A peak.

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. fs is the switching frequency (680kHz).

ΔIL

=

-V----I--N--

L

×

-D---

fS

(EQ. 4)

Table 1 gives typical values (worst case margins are

considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L,

FSWand IOMAX):

TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION

VIN

VO

L

IOMAX

(V)

(V)

(µH)

(mA)

5

9

6.8

2215

5

12

6.8

1673

5

15

6.8

1344

12

15

6.8

3254

12

18

6.8

2670

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. 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

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 recommended to match the

internal slope compensation as well as to maintain a good

transient response performance. The inductor must be able

to handle the average and peak currents expressed in

Equations 5 and 6:

ILAVG

=

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

1–D

ILPK

=

ILAVG

+

-Δ----I--L-

2

(EQ. 5)

(EQ. 6)

Some inductors are recommended in Table 3.

TABLE 3. BOOST INDUCTOR RECOMMENDATION

DIMENSIONS

INDUCTOR (mm) VENDOR

PART NUMBER

10µH/

5.1APEAK

5.9µH/

6APEAK

13x13x4.5 TDK

12.9X12.9X4 Sumida

RLF12545T-100M5R1

CDEP12D38NP-5R9MB-120

11

FN6367.1

February 21, 2008

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