(

)

(

)

´

-

-

GH(MAX)

AVDD

F

CE

V

= 2

V

2 × V

V

(

)

(

)

é

ù

´

-

´

-

´

ë

û

Q

AVDD

F

GH

GH

P =

2

V

2

V

V

I

´

D

GH

F

P = I

V

TPS65155

www.ti.com

SLVSB29

– JANUARY 2012

A current of the order of 1 mA through the feedback resistor network ensures good accuracy and increases the

circuit's immunity to noise. It also ensures a minimum load on the charge pump, which reduces output voltage

Note that the maximum voltage in an application is determined by the boost converter's output voltage and the

voltage drop across the diodes and PNP transistor. For a typical application in which the positive charge pump is

configured as a voltage doubler, the maximum output voltage is given by Equation 6.

(6)

collector-emitter voltage of the PNP transistor (recommended to be at least 1 V, to avoid transistor saturation).

Selecting the PNP Transistor (Positive Charge Pump)

The power dissipated in the transistor is given by Equation 7. The transistor must be able to dissipate this power

without its junction becoming too hot. Note that the ability to dissipate power depends heavily on adequate PCB

thermal design.

(7)

A pull-up resistor is also required between the transistor's base and emitter. The value of this resistor is not

critical, but it should be large enough not to divert significant current away from the base of the transistor. A

value of 100 k

Ω is suitable for most applications.

Selecting the Diodes (Positive Charge Pump)

Small-signal diodes can be used for most low current applications (

<50 mA) and higher rated diodes for higher

power applications. The average current through the diode is equal to the output current, so that the power

dissipated in the diode is given by Equation 8.

(8)

The peak current through the diode occurs during start-up and for a few cycles may be as high as a few amps.

However, this condition typically lasts for

<1 ms and can be tolerated by many diodes whose repetitive current

Table 3. Positive Charge Pump Diode Selection

PART NUMBER

COMPONENT SUPPLIER

BAV99W

150 mA

1 A for 1 ms

75 V

1 V at 50 mA

NXP

BAT54S

200 mA

600 mA for 1s

30 V

0.8 V at 100 mA

Fairchild Semiconductor

MBR0540

500 mA

5.5 A for 8 ms

40 V

0.51 at 500 mA

Fairchild Semiconductor

Selecting the Capacitors (Positive Charge Pump)

For lowest output voltage ripple, low-ESR ceramic capacitors are recommended. The actual value is not critical

and values in the range 1

µF to 10 µF are suitable for most applications. Larger capacitors provide better

performance in applications where large load transient currents are present.

A flying capacitor in the range 100 nF to 1

µF is suitable for most applications. Larger values experience a

smaller voltage drop by the end of each switching cycle, and allow higher output voltages and/or currents to be

achieved. Smaller values tend to be physically smaller and a bit cheaper. For best performance, it is

recommended to include a resistor of a few ohms (2

Ω is a good value to start with) in series with the flying

capacitor to limit peak currents occurring at the instant of switching.

Copyright

© 2012, Texas Instruments Incorporated

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