L6701TR datasheet(18/44 Pages) STMICROELECTRONICS

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8 Driver Section

L6701

18/44

Driver Section

The integrated high-current drivers allow using different types of power MOS (also multiple

MOS to reduce the equivalent RDS(on)), maintaining fast switching transition.

The drivers for the high-side MOSFETs use BOOTx pins for supply and PHASEx pins for

return. The drivers for the low-side MOSFETs use the VCC pin for supply and PGND pin for

return.

The controller embodies a anti-shoot-through and adaptive dead-time control to minimize low

side body diode conduction time maintaining good efficiency saving the use of Schottky diodes:

when the high-side MOSFET turns off, the voltage on its source begins to fall; when the voltage

reaches 2V, the low-side MOSFET gate drive is suddenly applied. When the low-side MOSFET

turns off, the voltage at LGATEx pin is sensed. When it drops below 1V, the high-side MOSFET

gate drive is suddenly applied. If the current flowing in the inductor is negative, the source of

high-side MOSFET will never drop. To allow the low-side MOSFET to turn-on even in this case,

a watchdog controller is enabled: if the source of the high-side MOSFET does not drop, the low

side MOSFET is switched on so allowing the negative current of the inductor to recirculate. This

mechanism allows the system to regulate even if the current is negative.

Power conversion input is flexible: 5V, 12V bus or any bus that allows the conversion (See

maximum duty cycle limitations) can be chosen freely.

8.1

Power Dissipation

L6701 embeds high current MOSFET drivers for both high side and low side MOSFETs: it is

then important to consider the power that the device is going to dissipate in driving them in

order to avoid overcoming the maximum junction operative temperature. In addition, since the

device has an exposed pad to better dissipate the power, the thermal resistance between

junction and ambient consequent to the layout is also important: thermal pad need to be

soldered to the PCB ground plane through several VIAs in order to facilitate the heat

dissipation.

Two main terms contribute in the device power dissipation: bias power and drivers' power.

●

Device Power (PDC) depends on the static consumption of the device through the supply

pins and it is simply quantifiable as follow (assuming to supply HS and LS drivers with the

same VCC of the device):

●

Drivers' power is the power needed by the driver to continuously switch on and off the

external MOSFETs; it is a function of the switching frequency and total gate charge of the

selected MOSFETs. It can be quantified considering that the total power PSW dissipated to

switch the MOSFETs (easy calculable) is dissipated by three main factors: external gate

resistance (when present), intrinsic MOSFET resistance and intrinsic driver resistance.

This last term is the important one to be determined to calculate the device power

dissipation. The total power dissipated to switch the MOSFETs results:

External gate resistors helps the device to dissipate the switching power since the same power

PSW will be shared between the internal driver impedance and the external resistor resulting in

a general cooling of the device.

CCDRx

⋅

BOO Tx

⋅

()

⋅

GHS

BO OT

⋅

GLS

CCDRx

⋅

()

⋅⋅

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