LT1936
APPLICATIO S I FOR ATIO
of this data sheet that show the maximum load current as
a function of input voltage and inductor value for several
popular output voltages. Low 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 mini-
mum inductance required to avoid subharmonic oscilla-
tions. 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 significant inductance due to long
wires or cables, additional bulk capacitance may be nec-
essary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply
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
U
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 filters 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 resis-
tance (ESR) and provide the best ripple performance. A
good value is:
W
U
U
C
OUT
=
150
V
OUT
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 tran-
sient performance will suffer. With an external compensa-
tion network, the loop gain can be lowered to compensate
for the lower capacitor value. When using the internal
compensation network, the lowest value for stable opera-
tion is:
C
OUT
>
66
V
OUT
1936fa
9
LINER [ LINEAR TECHNOLOGY ]
