LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 9

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

www.DataSheet4U.com
www.DataSheet4U.com
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:
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
showing the necessary R
frequency is in Figure 1.
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
effi ciency, 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 effi ciency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (f
given application can be calculated as follows:
where V
voltage, V
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V
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 fi nite
R1= R2
f
SW MAX
SWITCHING FREQUENCY (kHz)
(
IN
Figure 1. Switching Frequency vs. R
D
is the typical input voltage, V
) =
is the catch diode drop (~0.5V) and V
0.79V
V
OUT
200
300
400
500
t
ON MIN
(
– 1
) V
V
(
D
T
D
+ V
value for a desired switching
+ V
IN
OUT
/V
IN
OUT
– V
T
pin to ground. A table
ratio. Also, as shown
SW
R
)
OUT
T
T
VALUE (kΩ)
Value
SW(MAX)
88.7
68.1
187
121
is the output
SW
) for a
is the
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:
where f
minimum switch on time (~150ns), and the t
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
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.
where V
V
(~0.5V), V
load), f
t
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
pulse skipping operation where some switching pulses
ON(MIN)
OUT
DC
DC
V
IN MIN
MIN
MAX
is the output voltage, V
(
SW
SW
IN(MAX)
is the minimum switch on time (~150ns). Note that
= f
) =
= 1– f
SW
is the switching frequency, the t
is the switching frequency (set by R
SW
1– f
is the internal switch drop (~0.5V at max
t
is the maximum operating input voltage,
SW
ON MIN
V
IN
SW
OUT
(
t
and BOOST pins, and the operating
OFF MIN
t
OFF MIN
+ V
)
(
(
D
)
)
– V
D
is the catch diode drop
D
+ V
SW
LT1912
ON(MIN)
OFF(MIN)
T
), and
is the
9
1912f
is