AAT4901 Datasheet, PDF (9/16 Pages) Advanced Analogic Technologies – Buffered Power Full-Bridge

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Application Information
Input Supply Capacitor
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT4901 and
reduces the surge current drawn from the input power.
A 4.7μF to 10μF X7R or X5R low ESR/ESL ceramic capac-
itor is selected for the input supply decoupling. To mini-
mize the tray resistance, the capacitor should be placed
as closely as possible to the input pin. This keeps the
high frequency content of input current localized, mini-
mizing EMI and input voltage ripple.
Shoot-Through Protection
The internal high-side and low-side MOSFETs of the
AAT4901 cannot conduct at the same time to prevent
shoot-through current. When the high-side MOSFET
turns on, the low-side MOSFET turns off first; after 5ns
break-before-make time, the high-side MOSFET then
turns on. Similarly, before the low-side MOSFET turns
on, the high-side MOSFET turns off; after a certain
break-before-make time (5ns typ.), the low-side MOSFET
turns on. The dead time between the high-side and low-
side turn-on should be kept as low as possible to mini-
mize current flows through the body diode of the high-
side and/or low-side MOSFET(s). The break-before-make
shoot-through protection significantly reduces losses
associated with the driver at high frequency.
Thermal Calculations
In the dual low-side MOSFET driver application, the
power dissipation of the AAT4901 includes the power
dissipation in the MOSFETs due to charging and dis-
charging the gate capacitance, the AC quiescent current
power dissipation, and transient power in the driver dur-
ing output transitions. As the transient power is usually
very small, its losses can be ignored. Maximum package
power dissipation can be estimated by the following
equation:
Eq. 1: P_D(MAX) = V_CC · I_IN =
T_J(MAX) - T_A
θ_JA
= I_QAC · V_CC + Q_G(tot)F_SW · V_CC
PRODUCT DATASHEET
AAT4901
Buffered Power Full-Bridge
Where:
T_J(MAX) = junction temperature of the dice (°C).
T_A = ambient temperature (°C).
θ_JA = thermal resistance (225°C/W).
I_QAC = AC quiescent current of the driver (mA).
Q_G(tot) = total gate charge of external low side MOSFETs
(nC).
F_SW = switching frequency (MHz).
The maximum junction temperature for the SC70JW-8
package can be derived from Equation 1:
Eq. 2: T_J(MAX) = P_D(MAX) · θ_JA + T_A
For example, if the AAT4901 drives 2 AAT9560 MOSFETs
whose maximum gate charge is specified as 13nC for
V_GATE = 5V, the total power dissipation in the driver at a
switching frequency of 1MHz equals:
P_D(tot) = 2 · (5V · 13nC · 1MHz) + 5V · 4.0mA = 150mW
Gate Drive Current Ratings
Assuming that the maximum gate charge of the dual
low-side MOSFETs are equal, the maximum gate drive
capability for the designed maximum junction tempera-
ture without an external resistor can be derived from
Equation 1:
1
Eq. 3: Q_G(MAX) = _{2 · FSW} ·
T_J(MAX) - T_A
θ_JA · V_IN
- I_QAC
The relationship between gate capacitance, turn-on/
turn-off time, and the MOSFET driver current rating can
be determined by:
dV
Eq. 4: I_G(MAX) = C_G(MAX) · _dt
Where:
I_G(MAX) = peak drive current for a given voltage
C_G(MAX) = maximum gate capacitance
dV = MOSFET gate-to-source voltage
dt = rising time of MOSFET gate-to-source voltage
4901.2008.03.1.0
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