What is the max. gate charge that the TLE956x can drive?
One limiting factor is the current capability of the charge pump = 12 mA, (min value of ICPOC, parameter P_12_11_68/69).
- 3 half-bridges are controlled in PWM at the same time
- all MOSFETs are symmetrical (same gate charge)
- active freewheeling is enabled
- PWM frequency (FPWM) : 20 kHz
The gate of 3 MOSFETs in PWM and 3 freewheeling MOSFETs must be periodically charged.
The average current delivered by the charge pump is:
3 x 2 x QGMAX x FPWM = ICPOC
QGMAX = ICPOC / 6 FPWM
QGMAX = 12 mA / (6 x 20000) = 100 nC
Where QGMAX is the maximum gate charge for one MOSFET.
The factor 3 is for the number of half-bridges operating in PWM at the same time.
The factor 2 is for the PWM MOSFET and the freewheeling MOSFET of each half-bridge in PWM.
QGMAX = 100 nC is related to VGS = 11 V typ. for the TLE956x, whereas the QG of the MOSFET is in general specified at VGS = 10V.
Therefore the device can drive MOSFETs with a total gate charge of 90 nC at VGS = 10 V, when 3 half-bridges are simultaneously driven in PWM
The other limiting factor is the total power dissipation, which depends on the VCC1 current, on the ambient temperature and the thermal cooling of the package.
Note: if one half-bridge is activated without PWM, it must not be considered in the average current delivered by the charge pump.