Device description and operation

L6747C

8/15

Doc ID 17127 Rev 1

4

Device description and operation

The L6747C provides high-current driving control for both high-side and low-side N-channel

MOSFETs, connected as step-down DC-DC converters and driven by an external PWM

signal. The integrated high-current drivers allow the use of different types of power

transition. The driver for the high-side MOSFET uses the BOOT pin for supply and the

PHASE pin for return. The driver for the low-side MOSFET uses the VCC pin for supply and

the PGND pin for return.

The driver includes anti-shoot-through and adaptive dead-time control to minimize low-side

body diode conduction time, maintaining good efficiency and eliminating the need for

Schottky diodes. When the high-side MOSFET turns off, the voltage on its source begins to

fall; when the voltage falls below the proper threshold, the low-side MOSFET gate drive

voltage is suddenly applied. When the low-side MOSFET turns off, the voltage at the LGATE

pin is sensed. When it drops below the proper threshold, the high-side MOSFET gate drive

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

the high-side MOSFET never drops. 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, allowing the negative current of the inductor to

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

side MOSFETS firmly OFF. Then, after the UVLO has been crossed, the EN and PWM

inputs take control over the driver’s operations.

The EN pin enables the driver. If low, it keeps all MOSFETs OFF (HiZ) regardless of the

status of PWM. When EN is high, the PWM input takes control. If externally set within the

HiZ window, the driver enters an HiZ state and both MOSFETS are kept in an OFF state

until PWM exits the HiZ window (see Figure 4).

After the UVLO threshold has been crossed and while in HiZ, the preliminary OV protection

is activated. If the voltage sensed through the PHASE pin goes above about 1.8 V, the low-

side MOSFET is latched ON in order to protect the load from dangerous overvoltage. The

driver status is reset from a PWM transition.

Driver power supply, as well as power conversion input, are flexible: 5 V and 12 V can be

chosen for high-side and low-side MOSFET voltage drive.

Figure 4.

Timing diagram (EN = High)

HiZ Window

PWM

HS Gate

LS Gate

HiZ Window

V

PWM_IH

V

PWM_IL