SI-8090JF

Step-Down

to 9.0 V, 1.5 A,

DC/DC Converter

www.allegromicro.com

Switching

Regulators

5

APPLICATIONS INFORMATION

Input Capacitor (C1). Capacitors with low impedance

for high-frequency ripple current must be used.

Output Capacitor (C2). Capacitors with low impedance

for high-frequency ripple current must be used. Especially

when the C2 impedance is high, the switching waveform

may not be normal at low temperatures. Film or tantalum

capacitor for C2 may cause abnormal oscillations.

Catch Diode (D1). Diode D1 must be a Schottky diode.

Other diode types will result in increased forward voltage

spikes, reverse current flow, increased IC power dissipa-

tion during the off period, and possible destruction of the

IC.

Choke Coil (L1). If the winding resistance of the choke

coil is too high, the circuit efficiency will decrease. As the

overcurrent protection start current is approximately 2.5 A,

attention must be paid to the heating of the coil by magnetic

saturation due to overload. To reduce the output ripple, the

inductor may be increased at the expense of excessive

board area and cost.

Typical Application

SI-8090JF

Soft-Start Capacitor (C3). Soft start for the converter

is enabled by connecting a capacitor between terminal 5

and ground. The converter may be turned off by decreasing

the terminal 5 voltage below 0.5 V with either an npn

small-signal transistor or the output of open-collector TTL.

If both a large soft-start capacitor and on/off control are

desired, collector current limiting must be used to prevent

transistor damage. No external voltage can be applied to

terminal 5.

Parallel Operation. Parallel operation to increase load

current is not permitted.

Overcurrent Protection. The SI-8000JF series has a

built-in fold-back type overcurrent protection circuit, which

limits the output current at a start-up mode. It thus cannot

be used in applications that require current at the start-up

mode such as:

(1) constant-current load,

(2) power supply with positive and negative outputs to

common load (a center-tap type power supply), or

(3) raising the output voltage by putting a diode or a

resistor between the device ground and system ground.