L9908 datasheet(66/153 Pages) STMICROELECTRONICS

66 / 153 page

Figure 38. Pre-driver peak output current limitation circuit

The maximum PU/PD peak output currents can be estimated as follows:

IOUT_PU_PK=MINDRV_pu_iout, VHIGH

Rlim_on+RPU+Rg

(9)

IOUT_PD_PK=MINDRV_pu_iout, VHIGH−Vdfw

Rlim_on∥Rlim_off+RPD+Rg

(10)

Where: Vhigh is the driver`s output stage supply, Vdfw is the external diode forward voltage, Rg is the Ext. FET

series gate resistance, and Rlim_on/off are the external limitation resistances.

Example: if Rg = 1.6 Ω, Vdfw = 0.75 V, VHIGH = 10 V to have maximum peak current below 0.8 A the external

resistance shall be:IOUT_PU_PK= 10V

Rlim_on+1.15Ω+1.6Ω=0.8A Rlim_on+2.75Ω ≈12.5Ω

Rlim_on≈9.75Ω

(11)

IOUT_PD_PK= 10V−0.75V

Rlim_on Rlim_off+1.15Ω+1.6Ω=0.8A Rlim_on Rlim_off

+2.75Ω ≈11.56Ω 9.75Ω Rlim_off≈ 8.81Ω Rlim_on≈4.63Ω

(12)

5.12.1

Ext. FET VGS monitor

The correct functionality of each six pre-drivers is monitored by a dedicated comparator that senses the difference

between Gate and Source terminals of the Ext. FET.

If GHS_n-SHS_n (GLS_n-SLS_n) ≥ VGS_TH + VGS_uv_hys occurs for an interval longer than T_vgs_flt the VGS

comparator output goes to 0 and the HSn_OFF (LSn_OFF) flag is reset. The Ext FET is considered turned-on.

If GHS_n-SHS_n (GLS_n-SLS_n) ≤ VGS_TH occurs for an interval longer than T_vgs_flt the VGS comparator

output goes to 1 and the HSn_OFF (LSn_OFF) flag is set. The Ext FET is considered turned-off.

Table 60. VGS monitor electrical characteristics

Symbol

Parameter

Min

Typ

Max

Unit

Notes

VGS_mon_th

VGS monitor threshold

2.5

VGS_uv_hys

VGS monitor hysteresis

200

T_vgs_uv_flt

VGS monitor detection filter time

200

230

300

Analog filter

Note:

All parameters are guaranteed, and tested, in the voltage ranges reported above in Table 5 unless otherwise

specified. Where not specified the parametrical operating range equals the functional operating range.

L9908

Half bridges gate drivers

DS13546 - Rev 3

page 66/153

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