FDMF6840C datasheet(12/19 Pages) ONSEMI

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Functional Description

The FDMF6840C 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 Loc kout

(UVLO) c ircuit. When VCIN rises above ~3.1 V, the dr iver

is enabled. When VCIN falls below ~2.7 V, the driver is

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

pulling the DISB# pin LOW ( DISB# < VIL_DISB), w hich

holds both GL and GH LOW regardless of the PWM

input state. The dr iver 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-dow n current source is 10 µA.

Thermal Warning Flag (THWN#)

The FDMF6840C provides a ther mal w arning flag

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

ther mal w arning flag uses an open-drain output that

pulls to CGND w hen 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, w hich can be connected to

VCIN. THWN# does NOT disable the DrMOS module.

Figure 26.

THWN Operation

Three-State PWM Input

The FDMF6840C incorporates a three-state 3.3 V PWM

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

both logic HIGH level and LOW level, along w ith a

three-state shutdow n w indow . When the PWM input

signal enters and remains w ithin the three-state w indow

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

are pulled LOW. This enables the gate drive to shut

dow n both high-side and low -side MOSFETs to support

features such as phase shedding, w hich is common on

multi-phase voltage regulators.

Exiting Three-State Condition

When

exiting

valid

three-state

condition,

the

FDMF6840C follow s 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 27. The FDMF6840C

design allow s for short propagation delays w hen exiting

the three-state w indow (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 betw een the VDRV and

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

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

the driver is disabled (DISB#=0 V), 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

developed by

a bootstrap supply

circuit

consisting of the internal Schottky diode and external

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

at PGND, allow ing 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, w hich

provides sufficient VGS enhancement for Q1. To complete

the sw itching cycle, Q1 is turned off by pulling GH to

VSWH. CBOOT is then recharged to VDRV w hen VSWH falls to

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

high-side gate is held LOW w hen the driver is disabled or

the PWM signal is held w ithin the three-state w indow for

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

150°C

Activation

Temperature

TJ_driver IC

Ther mal

Warning

Nor mal

Operation

HIGH

LOW

135°C Reset

Temperature

THWN#

Logic

State

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