PRODUCT SPECIFICATION
RC4190
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.
The inductor value and oscillator frequency must be care-
fully tailored to the battery voltage, output current, and
ripple requirements of the application (refer to the Design
Equations Section). If the inductor value is too high or the
oscillator frequency is too high, then the inductor current
will never reach a value high enough to meet the load current
drain and the output voltage will collapse. If the inductor
value is too low or the oscillator frequency too low, then the
inductor current will build up too high, causing excessive
output voltage ripple, or over stressing of the switch transis-
tor, or possibly saturating the inductor.
voltage applied across the inductor will discharge into the
load. As in the step-up case, the average inductor current
equals the load current. The maximum inductor current
I
MAX
will equal (V
BAT
– V
OUT
)/L times the maximum on
time of the switch transistor (T
ON
). Current flows to the load
during both half cycles of the oscillator.
Complete Step-Down Regulator
Most step-down applications are better served by the
RC4391 step-down and inverting switching regulator (refer
to the RC4391 data sheet). However, there is a range of load
power for which the RC4190 has an advantage over the
RC4391 in step-down applications. From approximately 500
mW to 2W of load power, the RC4190 step-down circuit of
Figure 6 offers a lower component count and simpler circuit
than the comparable RC4391 circuit, particularly when step-
ping down a voltage greater than 30V.
Since the switch transistor in the RC4190 is in parallel with
the load, a method must be used to convert it to a series con-
nection for step-down applications. The circuit of Figure 11
accomplishes this. The 2N2907 replaces S of Figure 10,
and R6 and R7 are added to provide the base drive to the
2N2907 in the correct polarity to operate the circuit properly.
Simple Step-Down Converter
Figure 10 shows a step-down DC-to-DC Converter (V
OUT
≤
V
BAT
) with no feedback control.
S
L
(+)
Greater Than 30V Step-Down Regulator
V
BAT
D
C
R
L
V
OUT
(–)
65-1644
Figure 10. Simple Step-Down Converter
Adding a zener diode in series with the base of the 2N2907
allows the battery voltage to increase by the value of the
zener, with only a slight decrease in efficiency. As an exam-
ple, if a 24V zener is used, the maximum battery voltage can
go to 48V
2
when using a RC4190. Refer to Figure 12.
Notes:
1. The addition of the zener diode will not alter the maximum
change of supply. With a 24V zener, the circuit will stop
operating when the battery voltage drops below 24V +
2.2V = 26.2V.
2. Maximum battery voltage is 54V when using RM4190 (30V
+ 24V).
When S is closed, the battery voltage minus the output volt-
age is applied across the inductor. All of the inductor current
will flow into the load until the inductor current exceeds the
load current. The excess current will then charge the capaci-
tor and the output voltage will rise. When S is opened, the
2N2907
R6
Lx
V
OUT
D1
1N914
R7
5
R1
V
BAT
R4
6
1
+V
S
I
C
LBR
GND
3
C
X
2
Cx
R4 =
V
S
- 1.31V
5
µA
260K
~ 50
R6 ~
I
L
4190
4
L
X
V
FB
7
R3
R2
C
F
R5
65-2676
R5 =
10 V
S
~
R7 ~
I
L
Figure 11. Complete Step-Down Regulator
8
RAYTHEON [ RAYTHEON COMPANY ]
