Cole-Parmer Instrument Company
Manufactured by:
Barnant Company
Page
13 of 24
Waveform B is the voltage at point A in the schematic. It is at +V
in
for the time
Q1 is closed (T
on
) and at ground for the time Q1 is open (T- T
on
). The filter L2C2
averages out the V
in
peak-to-peak ripple voltage to produce a constant dc output
voltage V
o
whose value is given by:
V
o
= V
in
(T
on
/T)
The LM 2594 regulator continually adjusts T
on
to give a fixed +5Vdc output. The
period T remains fixed at a switching frequency of 150 KHz. The ripple voltage on V
o
is
determined by the choice of L2 and C2. Waveforms C through F show currents through
the switch Q1, diode D1, inductor L2, and capacitor C2. The average value of current
through the inductor L2 must always equal the dc load current. The current through L2
ramps upward through Q1 when Q1 is closed and ramps downward through D1 when
Q1 is open. The inductor current ramp must not be allowed to go to zero, placing a
minimum limit on the allowable load current. When Q1 is closed, there is a constant
voltage across L2 equal to V
in
- V
o
. Since the voltage across and inductor is given by V
= L di/dt, the magnitude of the peak of the current ramp is given by:
∆
I
L
= (V
in
- V
o
) T
on
/L2 .
Barnant Company
y
28W092 Commercial Avenue
y
Barrington, IL U.S.A. 60010-2392
847-381-7050
y
800-637-3739
y
847-381-7053 (Fax)
e-mail:
y
Web Site: http//www.barnant.com
