LT1936 Linear Technology, LT1936 Datasheet - Page 8

• Manufacturer Part Number: LT1936
• Description: 1.4A 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT1936
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
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
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:
where V
(~0.5V) and V
(~0.5V at maximum load). This leads to a minimum input
voltage of:
with DC
The maximum input voltage is determined by the absolute
maximum ratings of the V
minimum duty cycle DC
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
8
R1=R2
DC
V
V
IN MIN
IN MAX
(
(
=
IN
MAX
D
V
IN
is the forward voltage drop of the catch diode
)
and BOOST pins.
)
V
=
1.200
=
OUT
–
V
= 0.87.
OUT
V
SW
V
V
OUT
DC
SW
OUT
DC
+
is the voltage drop of the internal switch
MAX
– 1
V
+
MIN
+
D
+
V
V
V
D
D
D
MIN
–
–
IN
V
IN
V
D
D
= 0.08:
and BOOST pins.
and BOOST pins and by the
+
+
V
V
SW
SW
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is
where V
and L is in μH. With this value the maximum output cur-
rent will be above 1.2A at all duty cycles and greater than
1.4A for duty cycles less than 50% (V
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
effi ciency 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
Sumida
Of course, such a simple design guide will not always
result 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 effi ciency. Be aware
that if the inductance differs from the simple rule above,
then the maximum load current will depend on input volt-
age. There are several graphs in the Typical Performance
Characteristics section 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
L = 2.2 (V
D
URL
www.murata.com
www.component.tdk.com
www.toko.com
www.sumida.com
is the voltage drop of the catch diode (~0.4V)
OUT
+ V
D
)
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PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
IN
> 2 V
OUT
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
). The
1936fd