EL7641ILTZ datasheet(14/19 Pages) RENESAS

14 / 19 page

EL7640, EL7641, EL7642

FN7415 Rev 2.00

Page 14 of 19

February 22, 2006

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 EL7640, EL7641 and EL7642, 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) = 600



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

choose a convenient value greater than this minimum value;

say 700

. 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:

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:

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 the following equation:

where fOSC is the switching frequency.

Discontinuous/Continuous Boost Operation and its

Effect on the Charge Pumps

The EL7640, EL7641 and EL7642 VON and VOFF architecture

uses LX switching edges to drive diode charge pumps from

which LDO regulators generate the VON and VOFF supplies. It

VON

(>36V)

0.1µF

0.47µF

0.22µF

700



0.1µF

VBOOST

Q11

FBP

DRVP

EL7642

FIGURE 22. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP

NPOSITIVE

VOUT VCE VINPUT

–

VINPUT 2VF



–

--------------------------------------------------------------



NNEGATIVE

VOUTPUT VCE

VINPUT 2VF



–

-------------------------------------------------



COUT

IOUT

2VRIPPLE fOSC



------------------------------------------------------



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