DRV8701: Explain the Equation 10 in 8.2.3.2

Fandy

Expert 2285 points

Part Number: DRV8701

Dears

1.why there is demand of maximum voltage VM in EQ10 when VM is different from external H-Bridge supply?

2.Where is Equation 10 derive from ?

over 5 years ago

0 Rick Duncan over 5 years ago

TI__Guru** 114905 points

Hi Fandy,

1) The maximum voltage is to limit the maximum Vgs on the high side FET.

In the example of VBAT = 4V, if the DRV8701 VM is set to 20V the VCP voltage is ~30V. This would create a Vgs of ~26V, and may damage some FETs.

2) The 11.5V is the positive Vgs clamp voltage. The clamp is described in section 7.3.12 of the datasheet.

0 Fandy over 5 years ago in reply to Rick Duncan

Expert 2285 points

Hi Rick

Here is my understanding,

1、Described in 7.3.10,When VM<12V,VCP =2*VM -1.5; VCP provides High gate driver,so VG=VCP=2*VM-1.5, if HS MOS switched-off,VS = ? if HS MOS switched on,VS=Vbat, so VGS=VG-VS =2*VM-1.5-Vbat<VGS CALMP ,assume VGS CLAMP =10.5V, VGS=2*VM-1.5-Vbat<10.5 ; So VM<(12+Vbat)/2 ; it seems a little difference between with EQ 10.

2、Described in 7.3.10 ,If VM>12V,VCP=VM+9.5V. if HS MOS switched-off,VS = ? if HS MOS switched on,VS=Vbat, so VGS=VG-VS =VM+9.5-Vbat<VGS CALMP ,assume VGS CLAMP =10.5V ,VGS = VM+9.5V-Vbat<10.5V, So VM < 1+BAT ,it seems if VM>12V ,it must be trigger VGS CLAMP.

3、By the way,Described in 7.3.12,VGS trigger VGS CLAMP, The clamping diode will conducting to clamp voltage within 10.5-13V, only if the clamp diode was burn out,then maybe damage FETs

If above viewpoint is right ?and please help answer my question, thanks

0 Fandy over 5 years ago in reply to Rick Duncan

Expert 2285 points

Hi Rick

Blow is my understanding:

1、Described in 7.3.10, When VM<12V, VCP=2*VM - 1.5V ,Vcp provides HS Gate driver, VG=VCP=2*vm-1.5V, if HS MOS switch-off, VS=0 ? if HS MOS switched-on,VS=Vbat ,so VGS=VG-VS=2*VM-1.5-Vbat < VGS CLAMP, Assume VGS CLAMP=10.5V, So VGS=2*VM-1.5V-Vbat<10.5V , VM<(12+Vbat)/2, it seems a little differet from EQ10;

2、Described in 7.3.10,When VM>12V,VCP=VM+9.5 =VG,if HS MOS switch-off, VS=0 ? if HS MOS switched-on,VS=Vbat, VGS=VM+9.5-Vbat<VGS CLAMP ,so VM>12V, it is easy to trigger VGS CLAMP;

3、Described in 7.3.12, When VGS trigger VGS CLAMP, the Clamp Diode will conducting to clamp voltage to 10.5-13V, only if Clamp Diode was burn out, it maybe damage FETs.

if above viewpoint ids right ? and please answer my questioin,thanks

0 Rick Duncan over 5 years ago in reply to Fandy

TI__Guru** 114905 points

Hi Fandy,

Sorry for the delayed response.

1、Described in 7.3.10, When VM<12V, VCP=2*VM - 1.5V ,Vcp provides HS Gate driver, VG=VCP=2*vm-1.5V, if HS MOS switch-off, VS=0 ? if HS MOS switched-on,VS=Vbat ,so VGS=VG-VS=2*VM-1.5-Vbat < VGS CLAMP, Assume VGS CLAMP=10.5V, So VGS=2*VM-1.5V-Vbat<10.5V , VM<(12+Vbat)/2, it seems a little differet from EQ10;

>>> Equation 10 is a rough approximation. It is intended to be used considered when VBAT is below the minimum recommended operating voltage. The equation works until 11.5V. In most cases, there is no need to boost VM when operating above the minimum operating voltage. The Vgs will be sufficient to turn on the FETs (Vgs of 4V typical at VM=5.9V, Vgs of 6.4V at VM=8V).

2、Described in 7.3.10,When VM>12V,VCP=VM+9.5 =VG,if HS MOS switch-off, VS=0 ? if HS MOS switched-on,VS=Vbat, VGS=VM+9.5-Vbat<VGS CLAMP ,so VM>12V, it is easy to trigger VGS CLAMP;

>>> Equation 10 is not intended to be used above 11.5V. The device will provide Vgs of 9.5V typical.

3、Described in 7.3.12, When VGS trigger VGS CLAMP, the Clamp Diode will conducting to clamp voltage to 10.5-13V, only if Clamp Diode was burn out, it maybe damage FETs.

>>> Equation 10 is not intended to be used above 11.5V. The device will provide Vgs of 9.5V typical.

if above viewpoint ids right ? and please answer my questioin,thanks

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DRV8701: Explain the Equation 10 in 8.2.3.2