ISL97642 Просмотр технического описания (PDF) - Intersil

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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Intersil 

ISL97642

0.1µF

0.1µF

LX

VBOOST

DRVP

700Ω

Q11

0.1µF 0.1µF

ISL97642

0.47µF

0.1µF

VON

(>36V)

FBP

0.22µF

FIGURE 21. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP

Calculation of the Linear Regulator Base-emitter

Resistors (RBP and RBN)

For the pass transistor of the linear regulator, low frequency

gain (Hfe) and unity gain frequency (fT) are usually specified

in the datasheet. The pass transistor adds a pole to the loop

transfer function at fp = fT/Hfe. Therefore, in order to

maintain phase margin at low frequency, the best choice for

a pass device is often a high frequency, low gain switching

transistor. Further improvement can be obtained by adding a

base-emitter resistor RBE (RBP, RBL, RBN in the Functional

Block Diagram), which increases the pole frequency to:

fp = fT*(1+ Hfe *re/RBE)/Hfe, where re = KT/qIc. So choose

the lowest value RBE in the design as long as there is still

enough base current (IB) to support the maximum output

current (IC).

We will take as an example the VON linear regulator. If a

Fairchild MMBT3906 PNP transistor is used as the external

pass transistor (Q11 in the application diagram), then for a

maximum VON operating requirement of 50mA, the data

sheet indicates Hfe_min = 60. The base-emitter saturation

voltage is: Vbe_max = 0.7V.

For the ISL97642, the minimum drive current is:

I_DRVP_min = 2mA

The minimum base-emitter resistor, RBP, can now be

calculated as:

RBP_min= VBE_max ⁄ (I_DRVP_min - Ic/Hfe_min =

((0.7V) ⁄ (2mA – (50mA) ⁄ 60))= 600Ω

(EQ. 11)

This is the minimum value that can be used – so, we now

choose a convenient value greater than this minimum value;

for example, 700Ω. Larger values may be used to reduce

quiescent current, however, regulation may be adversely

affected by supply noise if RBP is made too high in value.

Charge Pump

To generate an output voltage higher than VBOOST, single or

multiple stages of charge pumps are needed. The number of

stage is determined by the input and output voltage. For

positive charge pump stages:

NPOSITIVE ≥ -V----O----VU----IT--N---+--P---VU----C-T---E-–----–-2----V-×---I-V-N----FP----U----T--

(EQ. 12)

where VCE is the dropout voltage of the pass component of

the linear regulator. It ranges from 0.3V to 1V depending on

the transistor selected. VF is the forward-voltage of the

charge-pump rectifier diode.

The number of negative charge-pump stages is given by:

NN

E

GA

T

I

V

E

≥

--V----O-----U----T---P----U-----T------+-----V----C----E--

VINPUT – 2 × VF

(EQ. 13)

To achieve high efficiency and low material cost, the lowest

number of charge-pump stages, which can meet the above

requirements, is always preferred.

Charge Pump Output Capacitors

Ceramic capacitor with low ESR is recommended. With

ceramic capacitors, the output ripple voltage is dominated by

the capacitance value. The capacitance value can be

chosen by Equation 14:

COUT ≥ 2-----×-----V----R-----I-P-I--O-P---U-L----ET-----×----f--O-----S----C--

(EQ. 14)

where fOSC is the switching frequency.

Discontinuous/Continuous Boost Operation and

its Effect on the Charge Pumps

The ISL97642 VON and VOFF architecture uses LX

switching edges to drive diode charge pumps from which

LDO regulators generate the VON and VOFF supplies. It can

be appreciated that should a regular supply of LX switching

14

FN6436.0

June 18, 2007

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