Package Lead Description
LEAD SYMBOL
PACKAGE LEAD #
FUNCTION
14 Lead PDIP
1
2
OUTPUT
Gnd
MOSFET Gate Drive
Ground
3
FLT
Fault time out capacitor
Oscillator capacitor
Oscillator resistor
Pulse width control input
No connection
4
COSC
ROSC
CTL
5
6
7
NC
8
VREG
ISENSE-
ISENSE+
IADJ
5V linear regulator
Current sense minus
Current sense plus
Current limit adjust
Output Inhibit
9
10
11
12
13
14
INH
PGnd
VCC
Power ground for on chip clamp
Positive power supply input
Application Information
I
ROSC is multiplied by two (2) internally and transferred to
Theory Of Operation
the COSC lead. Therefore:
Oscillator
The IC sets up a constant frequency triangle wave at the
COSC lead whose frequency is determined by the external
components ROSC and COSC by the following equation:
VCC
ICOSC = ±
ROSC
The period of the oscillator is:
0.83
Frequency =
ROSC ´ COSC
(VPEAK - VVALLEY
)
T = 2COSC
´
ICOSC
The peak and valley of the triangle wave are proportional
to VCC by the following:
VVALLEY = 0.2 ´ VCC
VPEAK = 0.8 ´ VCC
The ROSC and COSC components can be varied to create fre-
quencies over the range of 15Hz to 25kHz. With the sug-
gested values of 105k½ and 390pF for ROSC and COSC
respectively, the nominal frequency will be approximately
20 kHz. IROSC, at VCC = 14V, will be 66.7 µA. IROSC should
not change over a more than 2:1 ratio and therefore COSC
should be changed to adjust the oscillator frequency.
This is required to make the voltage compensation func-
tion properly. In order to keep the frequency of the oscilla-
tor constant the current that charges COSC must also vary
with supply. ROSC sets up the current which charges COSC
.
The voltage across ROSC is 50% of VCC and therefore:
Voltage Duty Cycle Conversion
The IC translates an input voltage at the CTL lead into a
duty cycle at the OUTPUT lead. The transfer function
incorporates Cherry SemiconductorÕs patented Voltage
Compensation method to keep the average voltage and
current across the load constant regardless of fluctuations
VCC
IROSC = 0.5 ´
ROSC
3
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
