CXA2534Q
c) Soft Start
During start-up of the IC, the duty cycle of the DC-to-DC converter is increased slowly to a maximum of
80 %. The VCC1 voltage will also increase slowly. This is known as soft start.
During the soft start, the duty cycle of the SW pin (pin 40) is dependent on the voltage at the DEAD pin
(pin 41). The DEAD pin is connected to a capacitor which is being charged up by an internal resistance
of 65 kΩ. Therefore, the time taken for the duty cycle to reach the maximum is
t = CSPRT × R
(sec)
where R = 65 kΩ
d) Power Off
To turn off the DC-to-DC Converter, a LOW level is applied to the OFF pin (pin 26). When the OFF pin
is low, the IC will not turn off the DC-to-DC converter immediately but will charge up the SPRT pin’s
capacitor with a constant current of 20 µA. Once the SPRT pin’s voltage is above 1.23 V, the IC will
then shut down. Therefore, the delay is given by the formula below:
1.23 V
t =
× CSPRT (sec)
20 µA
e) Over-Voltage Protection
The IC will detect a over-voltage if the voltage at the BSEN pin is above 8.4 V (typical). When over-
voltage occurred, the IC will shut down after a time delay. The time delay is determined by the time
taken for the SPRT pin’s capacitor to charge above 1.23 V. The charging current for the SPRT pin
capacitor is constant and is 20 µA.
1.23 V
t =
× CSPRT (sec)
20 µA
f) Reset Pulse
When the VCC1 voltage rises above 90 % of the designed value, the RESET pin (pin 25) will change
from LOW level to HIGH level. A hysteresis of 50 mV (typical) is implemented to prevent the ‘chattering’
of the output at RESET pin.
g) Empty Detection
The EMP pin (pin 24) is used to indicate low battery voltage. When the BSEN pin falls below a certain
threshold voltage, the EMP pin will change from HIGH to LOW level. There are two threshold voltages,
depending on the SEL pin (pin 37). In order to prevent ‘chattering’ of the output at the EMP pin, a
hysteresis of 50 mV (typical) is implemented.
Threshold Voltage
(typical)
Recovery Voltage
(typical)
SEL pin
LOW
2.0 V
2.05 V
High - Z
1.6 V
1.65 V
—21—
SONY [ SONY CORPORATION ]
