Atmel ATA6264 - Manual (Page 25)

25
4929B–AUTO–01/07
ATA6264 [Preliminary]
Figure 9-2. EVZ Regulator With Internal Divider
A draft formula for calculating the EVZ voltage, which is programmed by the external voltage
divider network at pin FBEVZ, is:
The pins EVZ and FBEVZ have to be shorted in applications without an external divider in order
to ensure a safe operation of the ATA6264 in the case of an EVZ-pin fault. If the voltage at pin
FBEVZ is larger than the voltage at pin EVZ, the ATA6264 switches the feedback path automat-
ically to pin FBEVZ. The remaining voltage at FBEVZ causes the regulator to switch off.
The output of the error amplifier is compared with a periodic linear ramp of a saw-tooth genera-
tor by the PWM comparator. A logic signal with variable pulse width is generated, which controls
the PWM frequency of the external FET. A maximum duty cycle is determined by the duration of
the falling ramp of the saw-tooth oscillator. The saw-tooth generator is controlled by the internal
100-kHz oscillator.
GNDA
-
+
-
+
Low battery
SPI
GEVZ
L
COMEVZO
FBEVZ
SPI
EVZ
OCEVZ
Logic and
driver
PWM
comp. Error
amp.
Bandgap
reference
Sawtooth oscillator
Overcurrent
EVZ
overvoltage
K30
Max. duty-cycle
SPI
C +
V
EVZ
V
REF
R
VZ1
R
VZ2
+
R
VZ2
-------------------------------- Ч
=