BiCMOS CURRENT MODE
PWM CONTROLLERS
TC18C43
TC28C43
TC38C43
PIN DESCRIPTION (Cont.)
Pin No.
8-Pin
Pin No
14-Pin
Pin No
16-Pin
Symbol
Description
6
7
8
NC
No Connection
No Connection
No Connection
13
NC
9
NC
POWER GROUND
ANALOG GND
OUTPUT
VDD
8
9
10
11
12
13
14
15
16
Ground return of output driver.
5
6
For all the low level analog signal returns.
Output to drive switching transistor gate input.
Supply power input terminal for the output drivers.
Voltage bias supply of all PWM Controller circuit functions.
Reference: 5.0 volt output.
10
11
12
14
7
8
VIN
VREF
NC
No Connection.
REFERENCE SECTION
1
fO ≈
(RT in Ohms and CT in Farads)
The reference is a zener-based design with a buffer
amplifier to drive the output. It is unstable with capacitances
between 0.01µF and 3.3µF. In a normal application a 4.7µF
is used. In some lower noise layouts the capacitor can be
eliminated entirely.
The reference is active as soon as the 38C4x has
power supplied. This is different than its bipolar counter-
parts, inthatthebipolarreferencecomesononlyaftertheIC
hascomeoutofitsundervoltagemode.Thus,onthe38C4x,
the reference pin can not be used as a reset function such
as on a soft start circuit.
RT CT
The value of RT affects the discharge current and the
upper and lower comparators each have delays. As RT gets
smaller and as the frequency of operation gets higher, the
above equation is no longer valid.
14
V
(5V)
REF
R
T
OSCILLATOR SECTION
7
9
The oscillator frequency is set by the combination of a
resistor from the reference to the RT/CT pin and by a
capacitor from this pin to ground. The oscillator is designed
to have ramp amplitude from 0.15 to 2.5 volts. This is
approximate, as over shoot on the oscillator comparator
causes the ramp amplitude to increase with frequency due
to comparator delay. Minimum values for CT and RT are
33pF and 1kΩ respectively. Maximum values are depen-
dent on leakage currents in the capacitor, not on the input
currents to the RT/CT pin.
R /C
T
T
C
T
GND
PIN NUMBERS FOR 14-PIN DIP
FREQUENCY OF OPERATION
The frequency of oscillation for the TC38C43 is con-
trolled by a resistor to VREF (RT) and a capacitor to ground
(CT).VREF suppliescurrentthroughtheresistorandcharges
the capacitor until its voltage reaches the threshold of the
upper comparator (≈2.5V). A 2.5mA current is then applied
to the capacitor to discharge it to near ground (≈0.15V). The
discharge current is then shut off and the cycle repeats. An
approximate equation for the frequency of operation is:
4-96
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
