L6238S datasheet(17/31 Pages) STMICROELECTRONICS

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4.3 Charge Pump

The charge pump circuitry is used as a means of

doubling the analog supply voltage in order to al-

low the upper N-channel DMOS transistors to be

driven like P-channel devices. The energy stored

in the reservoir capacitor is also used to drive the

lower drivers in a brake mode if the analog supply

is lost. Figure 4-3 is a simplified schematioc of the

charge pump circuitry.

A capacitor, Cpump, is used to retrieve energy

from the analog supply and then ”pumps” it into

the storage capacitor, Cresvr.

An internal 300KHz oscillator first turns ON Q2 to

quickly charge Cpump to approximately the rail

voltage. The oscillator then turns ON Q1 while

turning OFF Q2. Since the bottom plate of Cpump,

is now effectively at the rail voltage via D2.

A zener-referenced series-pass regulator supplies

Figure 4-3

Islew

PUMP

SW2

Cfet

UPPER A

VCTRL

VPOWER

Islew

SW3

Cfet

OUTPUT

OUTA

3.1V

SLEW RATE

REFERENCE CURRENT

RSLW

RSENSE

CSA

PWM SLEW/RC

LOWER A

D95IN315

VANALOG

Figure 4-4

L6238S

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