L6711 datasheet(39/50 Pages) STMICROELECTRONICS

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L6711

Driver section

39/50

Driver section

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

MOS to reduce the equivalent RdsON), 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 VCCDRx pin for supply and PGNDx pin for

return. A minimum voltage of 4.6V at VCCDRx pin is required to start operations of the

device. VCCDRx pins must be connected together.

The controller embodies a sophisticated anti-shoot-through system 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 turning on of the low-side mosfet even in this case, a watchdog controller

is enabled: if the source of the high-side mosfet doesn't drop for more than 240ns, 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.

The BOOTx and VCCDRx pins are separated from IC's power supply (VCC pin) as well as

signal ground (SGND pin) and power ground (PGNDx pin) in order to maximize the

switching noise immunity. The separated supply for the different drivers gives high flexibility

in mosfet choice, allowing the use of logic-level mosfet. Several combination of supply can

be chosen to optimize performance and efficiency of the application.

Power conversion input is also flexible; 5V, 12V bus or any bus that allows the conversion

(See maximum duty cycle limitations) can be chosen freely.

16.1

Power dissipation

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

The first one depends on the static consumption of the device through the supply pins and it

is simply quantifiable as follow:

PDC = VCC · (ICC + 3 · ICCDRx + 3 · IBOOTx)

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:

PSW = 3 · (QG_HS · VBOOT + QG_LS · VCCDR) · FSW

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.It is important to determine the device dissipated

power in order to avoid the junction working beyond its maximum operative temperature.

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