RC4190M datasheet(13/25 Pages) RAYTHEON

13 / 25 page

RC4190

PRODUCT SPECIFICATION

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).

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

improved by choosing transistors with low saturation volt-

ages and by using power Schottky diodes such as Motorola's

MBR030.

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.

Figure 18. Buck Boost Circuit (Step-Up/Down)

Figure 19. Step-Up Voltage Dependent Oscillator

+VBAT

65-2681

4190

100

µF

+VOUT

2N2906

2.2K

Equivalent

1N914

1.0 mH

+VS

GND

VFB

BAT

65-2683

4190

OUT

1N914

+VS

LBR

LBD

VFB

Gnd

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