7

LT1106

output goes high, enabling the oscillator. Switch Q1

alternately turns on causing current build-up in the induc-

tor; then turns off allowing the built-up current to flow into

the output capacitor via the catch diode. As the output

voltage increases, so does the voltage at C1’s negative

input. When it exceeds the reference voltage plus C1’s

hysteresis, C1 turns the oscillator off.

Switch current is limited to approximately 600mA by Q2,

R1 and C3. Two percent of Q1’s collector current flows in

Q2; this current flows through R1 causing a voltage drop

in R1 proportional to Q1’s collector current. When R1’s

drop equals 36mV, comparator C3 forces the oscillator

off. This action results in varying on-time, fixed off-time

operation that keeps peak switch current controlled. By

connecting a 0.1

µF capacitor from the Soft Start pin to

ground, a current will flow in Q3 upon start-up. The current

flows through 700

Ω resistor R2, reducing the amount of

current needed from Q2 to force the oscillator off. As

current flows into the 0.1

µF capacitor, the voltage at pin 2

increases and eventually current ceases to flow in Q3.

Inductor Selection

All components for use in PCMCIA Type I cards must be

less than 1.1mm high. This somewhat limits the selection

of appropriate inductors. Dale Electronics (605-665-9301)

manufactures the ILS-3825-01, a monolithic ferrite induc-

tor that meets Type I height requirements. Generally,

inductors used with the LT1106 must fulfill several re-

quirements. It must be able to carry 0.95A (the maximum

switch current) without saturation. DCR should be kept

low to maintain efficiency. The switching frequency of the

LT1106 is quite high, over 500kHz so magnetic material is

important. Ferrite core material works well in this fre-

quency range. Avoid low cost iron powder cores which

have substantial AC loss at the LT1106’s switching fre-

quency. Inductance value need not be over 10

µH.

Capacitor Selection

The LT1106 will operate with 1

µF of output capacitance.

Output ripple voltage is approximately 400mV with this

value and can be reduced significantly by increasing

output capacitance. The ripple voltage, although on the

high side, poses no problems for programming flash

memory. If operating the device in 5V ouput mode the

capacitance should be increased. Ceramic capacitors are

suitable for the output. Distributed capacitance, i.e.,

0.1

µF or 0.2µF units next to individual flash memory

chips, is acceptable. The input capacitor should have at

least some tantalum capacitance (low Q) to minimize

resonance on the input. Flash memory cards are typically

several inches away from a solid low impedance supply

due to sockets, connectors, etc. If just ceramic capaci-

tors are used at the supply pin of the LT1106, switching

currents will resonate the supply line causing ringing that

ceramic capacitors causes this. A few microfarad’s worth

of tantalum capacitors with moderate ESR and low Q

characteristics will reduce or eliminate the problem.

Diode Selection

As with inductors, most good power Schottky diodes are

in packages that exceed the 1.1mm height limit of the

Type I PCMCIA card. Motorola manufactures the MBRO530

Schottky diode, ideal for use with the LT1106. This

diode’s maximum height however, is 1.35mm, making it

difficult to use in Type 1 cards. Philips Components

manufactures the BAT54C. Four units in parallel make an

adequate diode.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.