F3
ICE3AS02 / ICE3AS02G / ICE3BS02 / ICE3BS02G
Functional Description
The Gate Driver is active low at voltages below the
3.4
PWM Section
undervoltage lockout threshold VVCCoff
.
0.72
PWM Section
Oscillator
VCC
Duty
Cycle
max
PWM-Latch
1
Clock
Gate
Z1
Gate
Soft Start
FF1
Q
Driver
Comparator
S
R
1
&
PWM
G8
Comparator
G9
Figure 8
Gate Driver
Current
Limiting
The driver-stage is optimized to minimize EMI and to
provide high circuit efficiency. This is done by reducing
the switch on slope when exceeding the external
Power Switch threshold. This is achieved by a slope
control of the rising edge at the driver’s output (see
Figure 9).
Gate
Figure 7
PWM Section
ca. t = 130ns
VGate
3.4.1
Oscillator
The oscillator generates
a fixed frequency. The
switching frequency for ICE3AS02/G is fOSC = 100kHz
and for ICE3BS02/G fOSC = 67kHz. A resistor, a
capacitor and a current source and current sink which
determine the frequency are integrated. The charging
and discharging current of the implemented oscillator
capacitor are internally trimmed, in order to achieve a
very accurate switching frequency. The ratio of
controlled charge to discharge current is adjusted to
reach a maximum duty cycle limitation of Dmax=0.72.
CLoad = 1nF
5V
t
Figure 9
Gate Rising Slope
Thus the leading switch on spike is minimized. When
the external Power Switch is switched off, the falling
shape of the driver is slowed down when reaching 2V
to prevent an overshoot below ground. Furthermore the
driver circuit is designed to eliminate cross conduction
of the output stage. During powerup when VCC is
below the undervoltage lockout threshold VVCCoff, the
output of the Gate Driver is low to disable power
transfer to the seconding side.
3.4.2
PWM-Latch FF1
The oscillator clock output provides a set pulse to the
PWM-Latch when initiating the external Power Switch
conduction. After setting the PWM-Latch can be reset
by the PWM comparator, the Soft Start comparator or
the Current-Limit comparator. In case of resetting, the
driver is shut down immediately.
3.4.3
Gate Driver
The Gate Driver is a fast totem pole gate drive which is
designed to avoid cross conduction currents and which
is equipped with a zener diode Z1 (see Figure 8) in
order to improve the control of the Gate attached power
transistors as well as to protect them against
undesirable gate overvoltages.
Version 1.1
10
21 May 2004
INFINEON [ Infineon ]
