AN1451 datasheet(9/43 Pages) STMICROELECTRONICS

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AN1451 APPLICATION NOTE

2.6 Charge pump external components

An internal oscillator, with its output at

CP pin, switches from GND to 10V with a typical frequency of 600kHz

(see Figure 7).

Figure 7. Charge Pump.

When the oscillator output is at ground, C5 is charged by VS through D2. When it rises to 10V, D2 is reverse

biased and the charge flows from C5 to C8 through D1, so the VBOOT pin, after a few cycles, reaches the max-

imum voltage of VS + 10V - VD1 - VD2, which supplies the high-side gate drivers.

With a differential voltage between VS and VBOOT of about 9V and both the bridges switching at 50kHz, the typ-

ical current drawn by the VBOOT pin is 1.85 mA.

Resistor

R4 is added to reduce the maximum current in the external components and to reduce the slew rate of

the rising and falling edges of the voltage at the

CP pin, in order to minimize interferences with the rest of the

circuit. For the same reason care must be taken in realizing the PCB layout of

R4, C5, D1, D2 connections (see

also the

Layout Considerations section). Recommended values for the charge pump circuitry are:

D1, D2

: 1N4148

: 100

Ω (1/8 W)

: 10nF 100V ceramic

: 220nF 25V ceramic

Due to the high charge pump frequency, fast diodes are required. Connecting the cold side of the bulk capacitor

(C8) to VS instead of GND the average current in the external diodes during operation is less than 10 mA (with

R4 = 100

Ω); at startup (when VS is provided to the IC) is less than 200 mA while the reverse voltage is about

10 V in all conditions. 1N4148 diodes withstand about 200 mA DC (1 A peak), and the maximum reverse voltage

is 75 V, so they should fit for the majority of applications.

L 6208

V S + 10 V - V D1

V S - V D1

f = 600 kH z

V SA

V SB

V BOOT

RDS(ON ) = 70

Ω

10 V

5 V

RDS(ON) = 70

Ω

To High -Side

Gate Dr ivers

10 V

f = 600 kHz

C h arge Pu m p

O scillator

V S + 10 V - V D1 - V D2

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