EL7585AILZ(2005) Просмотр технического описания (PDF) - Intersil

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производитель

EL7585AILZ
(Rev.:2005)

TFT-LCD Power Supply

Intersil 

EL7585A

Cascaded MOSFET Application

A 20V N-channel MOSFET is integrated in the boost

regulator. For the applications where the output voltage is

greater than 20V, an external cascaded MOSFET is needed

as shown in Figure 22. The voltage rating of the external

MOSFET should be greater than VBOOST.

VIN

VBOOST

LX

FB

EL7585A

FIGURE 22. CASCADED MOSFET TOPOLOGY FOR HIGH

OUTPUT VOLTAGE APPLICATIONS

Linear-Regulator Controllers (VON, VLOGIC, and

VOFF)

The EL7585A includes three independent linear-regulator

controllers, in which two are positive output voltage (VON

and VLOGIC), and one is negative. The VON, VOFF, and

VLOGIC linear-regulator controller functional diagrams,

applications circuits are shown in Figures 23, 24, and 25

respectively.

Calculation of the Linear Regulator Base-Emitter

Resistors (RBL, RBP and RBN)

For the pass transistor of the linear regulator, low frequency

gain (Hfe) and unity gain freq. (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 increase 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 VLOGIC linear regulator. If a

Fairchild FMMT549 PNP transistor is used as the external

pass transistor, Q5 in the application diagram, then for a

maximum VLOGIC operating requirement of 500mA the data

sheet indicates Hfe_min = 100.

The base-emitter saturation voltage is: Vbe_max = 1.25V

(note this is normally a Vbe ~ 0.7V, however, for the Q5

transistor an internal Darlington arrangement is used to

increase it's current gain, giving a 'base-emitter' voltage of

2 x VBE).

(Note that using a high current Darlington PNP transistor for

Q5 requires that VIN > VLOGIC + 2V. Should a lower input

voltage be required, then an ordinary high gain PNP

transistor should be selected for Q5 so as to allow a lower

collector-emitter saturation voltage).

For the EL7585A, the minimum drive current is:

I_DRVL_min = 8mA

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

calculated as:

RBL_min = VBE_max/(I_DRVL_min - Ic/Hfe_min) =

1.25V/(8mA - 500mA/100) = 417Ω

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

choose a convenient value greater than this minimum value;

say 500Ω. Larger values may be used to reduce quiescent

current, however, regulation may be adversely affected, by

supply noise if RBL is made too high in value.

VBOOST

LX

0.9V

PG_LDOP +

-

+

-

GMP

LDO_ON

0.1µF

36V

ESD

CLAMP

CP (TO 36V)

RBP

7kΩ

DRVP

FBP

0.1µF

Q3

VON (TO 35V)

RP1

RP2

20kΩ

CON

1: Np

FIGURE 23. VON FUNCTIONAL BLOCK DIAGRAM

11

FN7523.2

September 21, 2005

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