FDMF6704 Datasheet, PDF (5/13 Pages) Fairchild Semiconductor – The Xtra Small High Performance, High Frequency DrMOS Module

Description of Operation
Circuit Description
The FDMF6704 is a driver plus FET module optimized for
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.
PWM
When the PWM input goes high, the high side MOSFET turns
on. When it goes low, the low side MOSFET turns on. When it is
open, both the low side and high side MOFET will turn off.
The DISB# input is combined with the PWM signal to control the
driver output. In a typical multiphase design, DISB# will be a
shared signal used to turn on all phases. The individual PWM
signals from the controller will be used to dynamically enable or
disable individual phases.
Low-Side Driver
The low-side driver (LDRV) is designed to drive a ground
referenced low R_DS(ON) N-channel MOSFET. The bias for LDRV
is internally connected between VDRV and CGND. When the
driver is enabled, the driver's output is 180° out of phase with
the PWM input. When the driver is disabled (DISB# = 0 V),
LDRV is held low.
High-Side Driver
The high-side driver (HDRV) 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 diode and external bootstrap capacitor (C_BOOT). During
start-up, VSWH is held at PGND, allowing C_BOOT to charge to
VDRV through the internal diode. When the PWM input goes
high, HDRV will begin to charge the high-side MOSFET's gate
(Q1). During this transition, charge is removed from C_BOOT and
delivered to Q1's gate. As Q1 turns on, VSWH rises to V_IN,
forcing the BOOT pin to V_IN +V_C(BOOT), which provides
sufficient VGS enhancement for Q1. To complete the switching
cycle, Q1 is turned off by pulling HDRV to VSWH. C_BOOT is then
recharged to VDRV when VSWH falls to PGND. HDRV output is
in phase with the PWM input. When the driver is disabled, the
high-side gate is held low.
SMOD
The SMOD (Skip Mode) function allows for higher converter
efficiency under light load conditions. During SMOD, the LS
FET is disabled and it prevents discharging of output caps.
When the SMOD# pin is pulled high, the sync buck converter
will work in synchronous mode. When the SMOD# pin is pulled
low, the LS FET is turned off. The SMOD function does not have
internal current sensing. This SMOD# pin is connected to a
PWM controller which enables or disables the SMOD
automatically when the controller detects light load condition.
Normally this pin is Active Low.
Adaptive Gate Drive Circuit
The driver IC embodies an advanced design that ensures
minimum MOSFET dead-time while eliminating potential
shoot-through (cross-conduction) currents. It senses the state of
the MOSFETs and adjusts the gate drive, adaptively, to ensure
they do not conduct simultaneously. Refer to Figure 4 for the
relevant timing waveforms.
To prevent overlap during the low-to-high switching transition
(Q2 OFF to Q1 ON), the adaptive circuitry monitors the voltage
at the LDRV pin. When the PWM signal goes HIGH, Q2 will
begin to turn OFF after some propagation delay (t_PDLL). Once
the LDRV pin is discharged below 1 V, Q1 begins to turn ON
after adaptive delay t_DTHH.
To preclude overlap during the high-to-low transition (Q1 OFF to
Q2 ON), the adaptive circuitry monitors the voltage at the
VSWH pin. When the PWM signal goes LOW, Q1 will begin to
turn OFF after some propagation delay (t_PDHL). Once the
VSWH pin falls below 1 V, Q2 begins to turn ON after adaptive
delay t_DTLH.
Additionally, V_GS of Q1 is monitored. When V_GS(Q1) is
discharged low, a secondary adaptive delay is initiated, which
results in Q2 being driven ON after 250 ns_, regardless of VSWH
state. This function is implemented to ensure C_BOOT is
recharged each switching cycle, particularly for cases where the
power convertor is sinking current and VSWH voltage does not
fall below the 1 V adaptive threshold. The 250 ns secondary
delay is longer than t_DTLH.
FDMF6704 Rev. D
5
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