LT1106 Datasheet, PDF (7/8 Pages) Linear Technology – Micropower Step-Up DC/DC Converter for PCMCIA Card Flash Memory

LT1106
APPLICATIONS INFORMATION
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 0.1µF capacitor from the Soft Start pin to
ground, a current will flow in Q3 upon start-up. The current
flows through 700Ω resistor R2, reducing the amount of
current needed from Q2 to force the oscillator off. As
current flows into the 0.1µF 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 10µH.
Capacitor Selection
The LT1106 will operate with 1µF 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.,
0.1µF 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
can exceed 500mV_P-P. The high Q, low ESR nature of
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.
7