RC4391 Datasheet, PDF (8/22 Pages) Fairchild Semiconductor – Inverting and Step-Down Switching Regulator

PRODUCT SPECIFICATION
RC4391
When power is first applied, the output filter capacitor is dis-
charged so the ground lead potential starts at 0V. The refer-
ence voltage is forced to +1.25V above the ground lead and
pulls the feedback input (pin 8) more positive than the
ground lead. This positive voltage forces the control network
to begin pulsing the switch transistor. As the switching
action pumps up the output voltage, the ground lead rises
with the output until the voltage on the ground lead is equal
to the feedback voltage. At that point, the control network
reduces the time on time of the switch to maintain a constant
output.
This control network will vary the on time of the switch in
response to changes in load current or battery voltage (see
Figure 12). If the load current increases (waveform C), then
the transistor will remain on (waveformD) for a longer por-
tion of the oscillator cycle, (waveform B), thus allowing the
inductor current (waveform E) to build up to a higher peak
value. The duty cycle of the switch transistor varies in
response to changes in load and line.
Design Equations
The inductor value and timing capacitor (CX) value must be
carefully tailored to the input voltage, input voltage range,
output voltage, and load current requirements of the applica-
tion. The key to the problem is to select the correct inductor
value for a given oscillator frequency, such that the inductor
current rises to a high enough peak value (IMAX) to meet the
average load current drain. The selection of this inductor
value must take into account the variation of oscillator
frequency from unit to unit and the drift of frequency over
temperature. Use ±30% as a maximum variation of oscillator
frequency.
LBR
C2
Q2
LBD
V_REF
R1
V_FB
C1
R2
+1.25V
REF/Bias V_REF
C_X
A
Cx
B
OSC
+V_S
+Vs
D
GND
Q1
+V OUT
L_X
RC4391
E
F
Lx
C_F
R1
+V_OUT = (1.25V) ( _R2 )
D1
1N914
65-2475
Important Note: This circuit must have a minimum load ³ 1 mA always connected.
Figure 11. Step-Down Regulator – Standard Circuit
8