LT1936
9
1936fd
inductance may result in discontinuous mode operation,
which is okay but further reduces maximum load current.
For details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. Choosing L greater than 1.6
(V
OUT
+ V
D
) μH prevents subharmonic oscillations at all
duty cycles.
Catch Diode
A 1A Schottky diode is recommended for the catch diode,
D1. The diode must have a reverse voltage rating equal
to or greater than the maximum input voltage. The ON
Semiconductor MBRM140 is a good choice. It is rated
for 1A DC at a case temperature of 110°C and 1.5A at a
case temperature of 95°C. Diode Incorporated’s DFLS140L
is rated for 1.1A average current; the DFLS240L is rated
for 2A average current. The average diode current in an
LT1936 application is approximately I
OUT
(1 – DC).
Input Capacitor
Bypass the input of the LT1936 circuit with a 4.7μF or
higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and ap-
plied voltage, and should not be used. A 4.7μF ceramic
is adequate to bypass the LT1936 and will easily handle
the ripple current. However, if the input power source has
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT1936 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT1936 and the catch diode; see the
PCB Layout section. A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT1936. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1936 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1936’s
voltage rating. This situation is easily avoided; see the Hot
Plugging Safety section.
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT1936 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT1936 to ~3.7V.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated
by the LT1936 to produce the DC output. In this role it
determines the output ripple, and low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT1936’s control loop.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is:
C
V
OUT
OUT
=
150
where C
OUT
is in μF. Use X5R or X7R types. This choice
will provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor if the compensation network is also adjusted to
maintain the loop bandwidth.
A lower value of output capacitor can be used, but transient
performance will suffer. With an external compensation
network, the loop gain can be lowered to compensate for the
lower capacitor value. When using the internal compensa-
tion network, the lowest value for stable operation is:
C
V
OUT
OUT
>
66
APPLICATIONS INFORMATION
