ISL6614A Datasheet, PDF (9/12 Pages) Intersil Corporation – Dual Advanced Synchronous Rectified Buck MOSFET Drivers with Pre-POR OVP

ISL6614A
page 5 determine when the lower and upper gates are
enabled.
This feature helps prevent a negative transient on the output
voltage when the output is shut down, eliminating the
Schottky diode that is used in some systems for protecting
the load from reversed output voltage events.
In addition, more than 400mV hysteresis also incorporates
into the three-state shutdown window to eliminate PWM
input oscillations due to the capacitive load seen by the
PWM input through the body diode of the controller’s PWM
output when the power-up and/or power-down sequence of
bias supplies of the driver and PWM controller are required.
Power-On Reset (POR) Function
During initial startup, the VCC voltage rise is monitored.
Once the rising VCC voltage exceeds 9.8V (typically),
operation of the driver is enabled and the PWM input signal
takes control of the gate drives. If VCC drops below the
falling threshold of 7.6V (typically), operation of the driver is
disabled.
Pre-POR Overvoltage Protection
Prior to VCC exceeding its POR level, the upper gate is held
low and the lower gate is controlled by the overvoltage
protection circuits during initial startup. The PHASE is
connected to the gate of the low side MOSFET (LGATE),
which provides some protection to the microprocessor if the
upper MOSFET(s) is shorted during initial startup. For
complete protection, the low side MOSFET should have a
gate threshold well below the maximum voltage rating of the
load/microprocessor.
When VCC drops below its POR level, both gates pull low
and the Pre-POR overvoltage protection circuits are not
activated until VCC resets.
Internal Bootstrap Device
Both drivers feature an internal bootstrap Schottky diode.
Simply adding an external capacitor across the BOOT and
PHASE pins completes the bootstrap circuit. The bootstrap
function is also designed to prevent the bootstrap capacitor
from overcharging due to the large negative swing at the
trailing-edge of the PHASE node. This reduces voltage
stress on the boot to phase pins.
The bootstrap capacitor must have a maximum voltage
rating above UVCC + 5V and its capacitance value can be
chosen from Equation 1:
C _{B O O T _CAP}
≥
----------^Q-----^G----^A----^T---^E-----------
Δ V_{B O O T _CAP}
Q_GATE=
Q_{-----G-----1----}•_-----P_----V_-----C_----C_---
V_GS1
•
N_Q1
(EQ. 1)
where Q_G1 is the amount of gate charge per upper MOSFET
at V_GS1 gate-source voltage and N_Q1 is the number of control
MOSFETs per channel. The ΔV_{BOOT_CAP} term is defined as
the allowable droop in the rail of the upper gate drive.
As an example, suppose two IRLR7821 FETs are chosen as
the upper MOSFETs. The gate charge, Q_G, from the data
sheet is 10nC at 4.5V (V_GS) gate-source voltage. Then the
Q_GATE is calculated to be 53nC for PVCC = 12V. We will
assume a 200mV droop in drive voltage over the PWM
cycle. We find that a bootstrap capacitance of at least
0.267µF is required.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2 _20nC
Q_GATE = 100nC
50nC
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
ΔV_{BOOT_CAP} (V)
FIGURE 2. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
Gate Drive Voltage Versatility
The ISL6614A provides the user flexibility in choosing the
gate drive voltage for efficiency optimization. The ISL6614A
ties the upper and lower drive rails together. Simply applying
a voltage from 5V up to 12V on PVCC sets both gate drive
rail voltages simultaneously. Connecting a SOT-23 package
type of dual schottky diodes from the VCC to BOOT1 and
BOOT2 can bypass the internal bootstrap devices of both
upper gates so that the part can operate as a dual ISL6612
driver, which has a fixed VCC (12V typically) on the upper
gate and a programmable lower gate drive voltage.
Power Dissipation
Package power dissipation is mainly a function of the
switching frequency (f_SW), the output drive impedance, the
external gate resistance, and the selected MOSFET’s
internal gate resistance and total gate charge. Calculating
the power dissipation in the driver for a desired application is
critical to ensure safe operation. Exceeding the maximum
allowable power dissipation level will push the IC beyond the
maximum recommended operating junction temperature of
+125°C. The maximum allowable IC power dissipation for
the SO14 package is approximately 1W at room
temperature, while the power dissipation capacity in the
QFN packages, with an exposed heat escape pad, is around
2W. See “Layout Considerations” on page 10 for thermal
transfer improvement suggestions. When designing the
driver into an application, it is recommended that the
9
FN9160.4
May 5, 2008