PRODUCT SPECIFICATION
RC4190
Principles of Operation
Simple Step-Up Converter
The most common application, the step-up regulator, is
derived from a simple step-up (V
OUT
> V
BAT
) DC-to-EC
Converter (Figure 6).
L
D
(+)
If the switch is opened and closed repeatedly, at a rate much
greater than the time constant of the output RC, then a
constant dc voltage will be produced at the output.
An output voltage higher than the input voltage is possible
because of the high voltage produced by a rapid change of
current in the inductor. When the switch is opened, the
inductor voltage will instantly rise high enough to forward
bias the diode, to V
OUT
+ V
D
.
In the complete RC4190 regulator, a feedback control sys-
tem adjusts the on time of the switch, controlling the level of
inductor current, so that the average inductor discharge
current equals the load current, thus regulating the output
voltage.
V
BAT
S
C
R
L
V
OUT
(–)
65-1646
Figure 6. Simple Set-Up
When switch S is closed, the battery voltage is applied
across the inductor L. Charging current flows through the
inductor, building up a magnetic field, increasing as the
switch is held closed. While the switch is closed, the diode D
is reverse biased (open circuit) and current is supplied to the
load by the capacitor C. Until the switch is opened, the
inductor current will increase linearly to a maximum value
determined by the battery voltage, inductor value, and the
amount of time the switch is held closed (I
MAX
= V
BAT
/
L x T
ON
). When the switch is opened, the magnetic field
collapses, and the energy stored in the magnetic field is
converted into a discharge current which flows through the
inductor in the same direction as the charging current.
Because there is no path for current to flow through the
switch, the current must flow through the switch, the current
must flow through the diode to supply the load and charge
the output capacitor.
Complete Step-Up Regulator
A complete schematic of the minimum step-up application is
shown in Figure 7. The ideal switch in the DC-to-DC
Converter diagram is replaced by an open collector NPN
transistor Q1. C
F
functions as the output filter capacitor, and
D1 and L
X
replace D and L.
When power is first applied, the current in R1 supplies bias
current to pin 6 (I
C
). This current is stabilized by a unity gain
current source amplifier and then used as bias current for the
1.31V bandgap reference. A very stable bias current gener-
ated by the bandgap is mirrored and used to bias the remain-
der of the chip. At the same time the RC4190 is starting up,
current will flow through the inductor and the diode to
charge the output capacitor to V
BAT
– V
D
.
G
E
I
LX
L
X
R1
6
5
V
BAT
I
C
+V
S
Q1
B
OSC
GND
3
A
C
X
2
C
X
F
I
D
(+)
4
L
X
D
+
–
R
L
REF
LBR
1
NC
LBD
8
NC
R3
D1
RC4190
V
FB
+1.31V
7
R2
+
–
C
F
C
I
LOAD
V
OUT
= V
REF
( R2 + 1)
R3
(–)
65-2673A
Figure 7. Complete Step-Up Regulator
6
RAYTHEON [ RAYTHEON COMPANY ]
