ISL6566_06 Datasheet, PDF (17/29 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications

ISL6566
Advanced Adaptive Zero Shoot-Through Deadtime
Control (Patent Pending)
The integrated drivers incorporate a unique adaptive deadtime
control technique to minimize deadtime, resulting in high
efficiency from the reduced freewheeling time of the lower
MOSFET body-diode conduction, and to prevent the upper and
lower MOSFETs from conducting simultaneously. This is
accomplished by ensuring either rising gate turns on its
MOSFET with minimum and sufficient delay after the other has
turned off.
During turn-off of the lower MOSFET, the PHASE voltage is
monitored until it reaches a -0.3V/+0.8V trip point for a
forward/reverse current, at which time the UGATE is released
to rise. An auto-zero comparator is used to correct the r_DS(ON)
drop in the phase voltage preventing false detection of the
-0.3V phase level during r_DS(ON conduction period. In the case
of zero current, the UGATE is released after 35ns delay of the
LGATE dropping below 0.5V. During the phase detection, the
disturbance of LGATE falling transition on the PHASE node is
blanked out to prevent falsely tripping. Once the PHASE is
high, the advanced adaptive shoot-through circuitry monitors
the PHASE and UGATE voltages during a PWM falling edge
and the subsequent UGATE turn-off. If either the UGATE falls
to less than 1.75V above the PHASE or the PHASE falls to less
than +0.8V, the LGATE is released to turn on.
Internal Bootstrap Device
All three integrated 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 PHASE node. This reduces voltage
stress on the boot to phase pins.
The bootstrap capacitor must have a maximum voltage
rating above PVCC + 5V and its capacitance value can be
chosen from the following equation:
C _{B O O T _CAP}
≥
----------^Q-----^G----^A----^T---^E-----------
∆ V_{B O O T _CAP}
(EQ. 12)
Q_GATE=
Q_{-----G-----1----}•_-----P_----V_-----C_----C_---
V_GS1
•
N_Q1
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. The ∆V_{BOOT_CAP} term is defined as the
allowable droop in the rail of the upper gate drive.
1.6
1.4
1.2
1.
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
∆V_{BOOT_CAP} (V)
FIGURE 10. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
Gate Drive Voltage Versatility
The ISL6566 provides the user flexibility in choosing the
gate drive voltage for efficiency optimization. The controller
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.
Initialization
Prior to initialization, proper conditions must exist on the
ENLL, VCC, PVCC and the VID pins. When the conditions are
met, the controller begins soft-start. Once the output voltage is
within the proper window of operation, the controller asserts
PGOOD.
Enable and Disable
While in shutdown mode, the PWM outputs are held in a
high-impedance state. This forces the drivers to short gate-
to-source of the upper and lower MOSFET’s to assure the
MOSFETs remain off. The following input conditions must be
met before the ISL6566 is released from this shutdown
mode.
1. The bias voltage applied at VCC must reach the internal
power-on reset (POR) rising threshold. Once this
threshold is reached, proper operation of all aspects of
the ISL6566 is guaranteed. Hysteresis between the rising
and falling thresholds assure that once enabled, the
ISL6566 will not inadvertently turn off unless the bias
voltage drops substantially (see Electrical
Specifications).
17
FN9178.4
March 9, 2006