
PRODUCT SPECIFICATION
RC4191/RC4192/RC4193
6
Applications Discussion
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).
Figure 6. Simple Set-Up Converter
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 mag-
netic 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 cur-
rent must flow through the diode to supply the load and
charge the output capacitor.
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 RC4193 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.
D
(+)
(–)
V
OUT
L
BAT
R
C
S
L
V
65-1646
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 RC4193 is starting up,
current will flow through the inductor and the diode to
charge the output capacitor to V
BAT
– V
D
.
At this point, the feedback (pin 7) senses that the output volt-
age is too low, by comparing a division of the output voltage
(set by the ratio of R2 to R3) to the +1.31V reference. If the
output voltage is too low then the comparator output changes
to a logical zero. The NOR gate then effectively ANDs the
oscillator square wave with the comparator signal; if the
comparator output is zero AND the oscillator output is low,
then the NOR gate output is high and the switch transistor
will be forced on. When the oscillator goes high again, the
NOR gate output goes low and the switch transistor will turn
off. This turning on and off of the switch transistor performs
the same function that opening and closing the switch in the
simple DC-to-DC Converter does; i.e., it stores energy in the
inductor during the on time and releases it into the capacitor
during the off time.
The comparator will continue to allow the oscillator to turn
the switch on and off until enough charge has been delivered
to the capacitor to raise the feedback voltage above 1.31V.
Thereafter, this feedback system will vary the duration of the
on time in response to changes in load current or battery
voltage (see Figure 8). If the load current increases (wave-
form C), then the transistor will remain on (waveform D) for
a longer portion 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 time.