TC18C43 Datasheet, PDF (4/7 Pages) TelCom Semiconductor, Inc – BiCMOS CURRENT MODE PWM CONTROLLERS

TC18C43
TC28C43
TC38C43
BiCMOS CURRENT MODE
PWM CONTROLLERS
PIN DESCRIPTION (Cont.)
Pin No.
8-Pin
Pin No Pin No
14-Pin 16-Pin
Symbol
6
7
NC
13
8
NC
9
NC
8
10 POWER GROUND
5
9
11
ANALOG GND
6
10
12
OUTPUT
11
13
V_DD
7
12
14
V_IN
8
14
15
V_REF
16
NC
Description
No Connection
No Connection
No Connection
Ground return of output driver.
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.
No Connection.
REFERENCE SECTION
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, in that the bipolar reference comes on only after the IC
has come out of its under voltage mode. Thus, on the 38C4x,
the reference pin can not be used as a reset function such
as on a soft start circuit.
OSCILLATOR SECTION
The oscillator frequency is set by the combination of a
resistor from the reference to the R_T/C_T 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 C_T and R_T are
33pF and 1kΩ respectively. Maximum values are depen-
dent on leakage currents in the capacitor, not on the input
currents to the R_T/C_T pin.
f_O ≈ ¹ (R_T in Ohms and C_T in Farads)
R_T C_T
The value of R_T affects the discharge current and the
upper and lower comparators each have delays. As R_T gets
smaller and as the frequency of operation gets higher, the
above equation is no longer valid.
14
VREF _(5V)
RT
7
RT/CT
CT
9 _GND
PIN NUMBERS FOR 14-PIN DIP
FREQUENCY OF OPERATION
The frequency of oscillation for the TC38C43 is con-
trolled by a resistor to V_REF (R_T) and a capacitor to ground
(C_T). V_REF supplies current through the resistor and charges
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.