FDMF6708N datasheet(13/21 Pages) ONSEMI

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FDMF6708N • Rev. 1.0.3

Functional Description

The FDMF6708N is a driver-plus-FET module optimized

for the synchronous buck converter topology. A single

PWM input signal is all that is required to properly drive

the high-side and the low-side MOSFETs. Each part is

capable of driving speeds up to 1 MHz.

VCIN and Disable (DISB#)

The VCIN pin is monitored by an Under-Voltage Lockout

(UVLO) circuit. When VCIN rises above ~3.3 V, the driver

is enabled. When VCIN falls below ~2.95 V, the driver is

disabled (GH, GL=0). The driver can also be disabled by

pulling the DISB# pin LOW (DISB# < VIL_DISB), which

holds both GL and GH LOW regardless of the PWM

input state. The driver can be enabled by raising the

DISB# pin voltage HIGH (DISB# > VIH_DISB).

Table 1.

UVLO and Disable Logic

UVLO

DISB#

Driver State

Disabled (GH, GL=0)

Enabled (see Table 2)

Open

Disabled (GH, GL=0)

Note:

DISB# internal pull-down current source is 10 µA.

Thermal Warning Flag (THWN#)

The FDMF6708N provides a thermal warning flag

(THWN#) to warn of over-temperature conditions. The

thermal warning flag uses an open-drain output that

pulls to CGND when the activation temperature (150°C)

is reached. The THWN# output returns to a high-

impedance state once the temperature falls to the reset

temperature (135°C). For use, the THWN# output

requires a pull-up resistor, which can be connected to

VCIN. THWN# does NOT disable the DrMOS module.

Figure 28.

THWN Operation

Three-State PWM Input

The FDMF6708N incorporates a three-state 5 V PWM

input gate drive design. The three-state gate drive has

both logic HIGH level and LOW level, along with a

three-state shutdown window. When the PWM input

signal enters and remains within the three-state window

for a defined hold-off time (tD_HOLD-OFF), both GL and GH

are pulled LOW. This enables the gate drive to shut

down both high-side and low-side MOSFETs to support

features such as phase shedding, which is common on

multi-phase voltage regulators.

Exiting Three-State Condition

When exiting a valid three-state condition, the

FDMF6708N follows the PWM input command. If the

PWM input goes from three-state to LOW, the low-side

MOSFET is turned on. If the PWM input goes from

three-state to HIGH, the high-side MOSFET is turned

on. This is illustrated in Figure 29. The FDMF6708N

design allows for short propagation delays when exiting

the three-state window (see Electrical Characteristics).

Low-Side Driver

The low-side driver (GL) is designed to drive a ground-

referenced, low-RDS(ON), N-channel MOSFET. The bias

for GL is internally connected between the VDRV and

CGND pins. When the driver is enabled, the driver's

output is 180° out of phase with the PWM input. When

the driver is disabled (DISB#=0V), GL is held LOW.

High-Side Driver

The high-side driver (GH) is designed to drive a floating

N-channel MOSFET. The bias voltage for the high-side

driver is developed by a bootstrap supply circuit

consisting of the internal Schottky diode and external

bootstrap capacitor (CBOOT). During startup, VSWH is held

at PGND, allowing CBOOT to charge to VDRV through the

internal diode. When the PWM input goes HIGH, GH

begins to charge the gate of the high-side MOSFET (Q1).

During this transition, the charge is removed from CBOOT

and delivered to the gate of Q1. As Q1 turns on, VSWH

rises to VIN, forcing the BOOT pin to VIN + VBOOT, which

provides sufficient VGS enhancement for Q1. To complete

the switching cycle, Q1 is turned off by pulling GH to

VSWH. CBOOT is then recharged to VDRV when VSWH falls to

PGND. GH output is in-phase with the PWM input. The

high-side gate is held LOW when the driver is disabled or

the PWM signal is held within the three-state window for

longer than the three-state hold-off time, tD_HOLD-OFF.

150°C

Activation

Temperature

TJ_driver IC

Thermal

Warning

Normal

Operation

HIGH

LOW

135°C Reset

Temperature

THWN#

Logic

State

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