RC4190
PRODUCT SPECIFICATION
improved by choosing transistors with low saturation volt-
ages and by using power Schottky diodes such as Motorola's
MBR030.
Buck-Boost Circuit (Step-Up/Down)
A disadvantage of the standard step-up and step-down
circuits is the limitation of the input voltage range; for a step-
up circuit, the battery voltage must always be less than the
programmed output voltage, and for a step-down circuit, the
battery voltage must always be greater than the output volt-
age. The following circuit eliminates this disadvantage,
allowing a battery voltage above the programmed output
voltage to decay to well below the output voltage (see
Figure 18).
Step-Up Voltage Dependent Oscillator
The RC4190's ability to supply load current at low battery
voltages depends on the inductor value and the oscillator
frequency. Low values of inductance or a low oscillator
frequency will cause a higher peak inductor current and
therefore increase the load current capability. A large induc-
tor current is not necessarily best, however, because the large
amount of energy delivered with each cycle will cause a
large voltage ripple at the output, especially at high input
voltages. This trade-off between load current capability and
output ripple can be improved with the circuit connection
shown in Figure 19. This circuit uses the low battery detector
to sense for a low battery voltage condition and will decrease
the oscillator frequency after a pre-programmed threshold is
reached.
The circuit operation is similar to the step-up circuit opera-
tion, except that both terminal of the inductor are connected
to switch transistors. This switching method allows the
inductor to be disconnected from the battery during the time
the inductor is being discharged. A new discharge path is
provided by D1, allowing the inductor to be referenced to
ground and independent of the battery voltage. The effi-
ciency of this circuit will be reduced to 55-60% by losses in
the extra switch transistor and diode. Efficiency can be
+VBAT
D2
1N914
D1
1N914
1.0 mH
+VOUT
L
x
2N2906
or
Equivalent
CF
100 µF
R4
2.2K
R1
1M
5
6
R2
IC
+VS
4
LX
4190
7
VFB
CX
GND
3
2
R3
C
x
65-2681
Figure 18. Buck Boost Circuit (Step-Up/Down)
L
x
+VBAT
1N914
+VOUT
R1
1M
6
CF
R2
5
4
R4
R5
IC
+VS
LX
VFB
1
7
LBR
4190
LBD
8
CX
Gnd
R3
3
2
C2
65-2683
C
x
Figure 19. Step-Up Voltage Dependent Oscillator
13
RAYTHEON [ RAYTHEON COMPANY ]
