LT1912
9
1912fa
APPLICATIONS INFORMATION
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:
R1= R2
V
OUT
0.79V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT1912 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 500kHz
by using a resistor tied from the R
T
pin to ground. A table
showing the necessary R
T
value for a desired switching
frequency is in Figure 1.
SWITCHING FREQUENCY (kHz) R
T
VALUE (kΩ)
200
300
400
500
187
121
88.7
68.1
Figure 1. Switching Frequency vs R
T
Value
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
SW MAX
( )
=
V
D
+ V
OUT
t
ON MIN
( )
V
D
+ V
IN
– V
SW
( )
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V) and V
SW
is the
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
IN
/V
OUT
ratio. Also, as shown
in the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT1912 switch has
finite minimum on and off times. The switch can turn on
for a minimum of ~150ns and turn off for a minimum of
~150ns. Typical minimum on time at 25°C is 80ns. This
means that the minimum and maximum duty cycles are:
DC
MIN
f
SW
t
ON MIN
( )
DC
MAX
= 1– f
SW
t
OFF MIN
( )
where f
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on time (~150ns), and the t
OFF(MIN)
is
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT1912 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V
IN
and BOOST pins, and the operating mode.
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following equation.
V
IN MIN
( )
=
V
OUT
V
D
1– f
SW
t
OFF MIN
( )
– V
D
+ V
SW
where V
IN(MAX)
is the maximum operating input voltage,
V
OUT
is the output voltage, V
D
is the catch diode drop
(~0.5V), V
SW
is the internal switch drop (~0.5V at max
load), f
SW
is the switching frequency (set by R
T
), and
t
ON(MIN)
is the minimum switch on time (~150ns). Note that
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 36V are acceptable regardless of the
switching frequency. In this mode, the LT1912 may enter
