LT1936
APPLICATIO S I FOR ATIO
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1%
resistors according to:
⎛
V
⎞
R1
=
R2
⎜
OUT
– 1
⎟
⎝
1.200
⎠
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for LT1936 applications depends
on the output voltage and the Absolute Maximum Ratings
of the V
IN
and BOOST pins.
The minimum input voltage is determined by either the
LT1936’s minimum operating voltage of ~3.45V or by its
maximum duty cycle. The duty cycle is the fraction of time
that the internal switch is on and is determined by the input
and output voltages:
DC
=
V
OUT
+
V
D
V
IN
– V
SW
+
V
D
where V
D
is the forward voltage drop of the catch diode
(~0.5V) and V
SW
is the voltage drop of the internal switch
(~0.5V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
=
V
OUT
+
V
D
– V
D
+
V
SW
DC
MAX
with DC
MAX
= 0.87.
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BOOST pins and by the
minimum duty cycle DC
MIN
= 0.08:
V
IN(MAX)
=
V
OUT
+
V
D
– V
D
+
V
SW
DC
MIN
Note that this is a restriction on the operating input
voltage; the circuit will tolerate transient inputs up to the
absolute maximum ratings of the V
IN
and BOOST pins.
8
U
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is
L = 2.2 (V
OUT
+ V
D
)
where V
D
is the voltage drop of the catch diode (~0.4V) and
L is in
µH.
With this value the maximum output current will
be above 1.2A at all duty cycles and greater than 1.4A for
duty cycles less than 50% (V
IN
> 2 V
OUT
). The inductor’s
RMS current rating must be greater than the maximum
load current and its saturation current should be about
30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 2.6A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Table 1. Inductor Vendors
VENDOR
Murata
TDK
Toko
URL
www.murata.com
www.component.tdk.com
www.toko.com
PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
Sumida
www.sumida.com
W
U
U
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value provides a slightly higher maximum load current and
will reduce the output voltage ripple. If your load is lower
than 1.2A, then you can decrease the value of the inductor
and operate with higher ripple current. This allows you to
use a physically smaller inductor, or one with a lower DCR
resulting in higher efficiency. Be aware that if the inductance
differs from the simple rule above, then the maximum load
current will depend on input voltage. There are several
graphs in the Typical Performance Characteristics section
1936fa
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