ADP3415LRM-REEL AD [Analog Devices], ADP3415LRM-REEL Datasheet - Page 8

• Manufacturer Part Number: ADP3415LRM-REEL
• Description: Dual MOSFET Driver with Bootstrapping
• Manufacturer: AD [Analog Devices]

ADP3415
THEORY OF OPERATION
The ADP3415 is a dual MOSFET driver optimized for driving
two N-channel FETs in a synchronous buck converter topology.
A single duty ratio modulation signal is all that is required to
command the proper drive signal for the high-side and the
low-side FETs.
A more detailed description of the ADP3415 and its features
follows. Refer to the Functional Block Diagram (Figure 2).
Drive State Input
The drive state input, IN, should be connected to the duty ratio
modulation signal of a switch-mode controller. IN can be driven
by 2.5 V to 5.0 V logic. The FETs will be driven so that the SW
node follows the polarity of IN.
Low-Side Driver
The supply rails for the low-side driver, DRVL, are VCC and
GND. In its conventional application, it drives the gate of the
synchronous rectifier FET.
When the driver is enabled, the driver’s output is 180° out of
phase with the duty ratio input aside from overlap protection
circuit, propagation, and transition delays. When the driver is
shut down or the entire ADP3415 is in shutdown or in under-
voltage lockout, the low-side gate is held low.
High-Side Driver
The supply rail for the high-side driver, DRVH, is between the
BST and SW pins and is created by an external bootstrap sup-
ply circuit. In its conventional application, it drives the gate of
the (top) main buck converter FET.
The bootstrap circuit comprises a Schottky diode, D
bootstrap capacitor, C
the SW pin is at ground, so the bootstrap capacitor will charge
up to VCC through DBST. As the supply voltage ramps up and
exceeds the UVLO threshold, the driver is enabled. When the
input pin, IN, goes high, the high-side driver will begin to turn
the high-side FET (Q1) ON by transferring charge from C
the gate of the FET. As Q1 turns ON, the SW pin will rise up to
V
gate to source voltage to hold Q1 ON. To complete the cycle,
when IN goes low, Q1 is switched OFF as DRVH discharges
the gate to the voltage at the SW pin. When the low-side FET,
Q2, turns ON, the SW pin is held at ground. This allows the
bootstrap capacitor to charge up to VCC again.
The high-side driver’s output is in phase with the duty ratio
input. When the driver is in undervoltage lockout, the high-side
gate is held low.
Overlap Protection Circuit
The overlap protection circuit (OPC) prevents both of the main
power switches, Q1 and Q2, from being ON at the same time.
This prevents excessive shoot-through currents from flowing
through both power switches and minimizes the associated
losses that can occur during their ON-OFF transitions. The
overlap protection circuit accomplishes this by adaptively
controlling the delay from Q1’s turn OFF to Q2’s turn ON and
by programming the delay from Q2’s turn OFF to Q1’s turn ON.
To prevent the overlap of the gate drives during Q1’s turn OFF
and Q2’s turn ON, the overlap circuit monitors the voltage at
the SW pin. When IN goes low, Q1 will begin to turn OFF
(after a propagation delay), but before Q2 can turn ON, the
DCIN
, forcing the BST pin to V
BST
. When the ADP3415 is starting up,
DCIN
+ V
C(BST)
, which is enough
BST
, and
BST
to
–8–
overlap protection circuit waits for the voltage at the SW pin to
fall from V
fallen to 1.6 V, Q2 will begin to turn ON. By waiting for the
voltage on the SW pin to reach 1.6 V, the overlap protection
circuit ensures that Q1 is OFF before Q2 turns on, regardless of
variations in temperature, supply voltage, gate charge, and drive
current. There is, however, a timeout circuit that will override
the waiting period for the SW pin to reach 1.6 V. After the
timeout period has expired, DRVL will be asserted regardless of
the SW voltage.
To prevent the overlap of the gate drives during Q2’s turn OFF
and Q1’s turn ON, the overlap circuit provides a programmable
delay that is set by a resistor on the DLY pin. When IN goes
high, Q2 will begin to turn OFF (after a propagation delay), but
before Q1 can turn ON, the overlap protection circuit waits for
the voltage at DRVL to go low. Once the voltage at DRVL is
low, the overlap protection circuit initiates a delay timer that is
programmed by the external resistor R
adds an additional specified delay. The delay allows time for
current to commutate from the body diode of Q2 to an external
Schottky diode, which allows turn-off losses to be reduced.
Although not as foolproof as the adaptive delay, the program-
mable delay adds a safety margin to account for variations in size,
gate charge, and internal delay of the external power MOSFETs.
Low-Side Driver Shutdown
The low-side driver shutdown, DRVLSD, allows a control
signal to shut down the synchronous rectifier. This signal should
be modulated by system state logic to achieve maximum battery
life under light load conditions and maximum efficiency under
heavy load conditions. Under heavy load conditions, DRVLSD
should be high so that the synchronous switch is modulated for
maximum efficiency. Under light load conditions, DRVLSD
should be low to prevent needless switching losses due to charge
shuttling caused by polarity reversal of the inductor current
when the average current is low.
When the DRVLSD input is low, the low-side driver stays low.
When the DRVLSD input is high, the low-side driver is enabled
and controlled by the driver signals as previously described.
Low-Side Driver Timeout Circuit
In normal operation, the DRVH signal tracks the IN signal
and turns OFF the Q1 high-side switch with a few tens of ns
tpdl
When Q1 is turned OFF, then DRVL is allowed to go high,
Q2 to turn ON, and the SW node voltage to collapse to zero.
But in a faulty scenario, such as the case of a high-side Q1
switch drain-source short circuit when even DRVH goes low,
the SW node cannot fall to zero.
The ADP3415 has a timer circuit to address this scenario. Every
time the IN goes low, a DRVL on-time delay timer gets trig-
gered (see Figure 2). Should the SW node voltage not trigger
the low side turn-on, the DRVL on-time delay circuit will do it
instead, when it times out with t
high-side Q1 is still turned ON, i.e., its drain is shorted to the
source, the low-side Q2 turn-on will create a direct short circuit
across the V
the fuse in the V
the load (CPU) from potential damage that the high-side switch
short circuit could have caused.
DRVH
delay following the falling edge of the input signal.
DCIN
DCIN
DCIN
to 1.6 V. Once the voltage on the SW pin has
voltage rail, and the crowbar action will blow
current patch. The opening of the fuse saves
SWTO
delay (see Figure 5). If the
DLY
. The delay resistor
REV. B