AN4370 datasheet(17/23 Pages) STMICROELECTRONICS

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DocID025336 Rev 1

17/23

AN4370

Estimating power dissipation

The power dissipation PD of the L99ASC03 depends on use conditions and external

components. Considering the main contributions, we can write:

PD =PVDD + PREG + PCP + PDRV

where

PVDD: power dissipation of the VDD voltage regulator

PREG: power dissipation of the internal voltage regulator

PCP: power dissipation of the charge pump

PDRV: power dissipation of the gate drivers

PVDD =(VVSREG - VVDD) x IVDD , with VVDD =5V

PREG =VVSREG x IVSREG , with IVSREG =25mA max

As to PCP and PDRV, no formula can be used to calculate their contribution. Application-

related parameters have a direct influence on these terms, such as the total gate charge of

the external MOSFETs (QG), the PWM frequency used to drive the motor (fPWM) and the

number of MOSFETs switched in a PWM period (N). Using external gate resistors will

reduce the power dissipation in the L99ASC03.

As a worst-case scenario, PCP and PDRV can be estimated by using Figure 10. The curves

in Figure 10 have been measured on one L99ASC03 unit at room temperature.

Figure 10. Supply current at VS pin vs. input supply voltage VVS as a function of charge pump

load (CCP1 =CCP2 = 220 nF, CTANK =1 µF)

AN4370 Datasheet(PDF) 17 Page - STMicroelectronics