LT3493EDCB View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT3493EDCB Datasheet PDF : 20 Pages
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LT3493
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
VOUT
0.78V
–
1
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
An optional phase lead capacitor of 22pF between VOUT
and FB reduces light-load output ripple.
Input Voltage Range
The input voltage range for LT3493 applications depends
on the output voltage and on the absolute maximum rat-
ings of the VIN and BOOST pins.
The minimum input voltage is determined by either the
LT3493’s minimum operating voltage of 3.6V, 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 = VOUT + VD
VIN – VSW + VD
where VD is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.4V at maximum load). This leads to a minimum input
voltage of:
VIN(MIN)
=
VOUT + VD
DCMAX
–
VD
+
VSW
with DCMAX = 0.91 (0.88 over temperature).
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins. For con-
tinuous mode operation, the maximum input voltage is
determined by the minimum duty cycle DCMIN = 0.10:
VIN(MAX )
=
VOUT + VD
DCMIN
–
VD
+ VSW
Note that this is a restriction on the operating input voltage
for continuous mode operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings
8
of the VIN and BOOST pins. The input voltage should be
limited to the VIN operating range (36V) during overload
conditions (short-circuit or start-up).
Minimum On Time
The part will still regulate the output at input voltages that
exceed VIN(MAX) (up to 40V), however, the output voltage
ripple increases as the input voltage is increased. Figure 1
illustrates switching waveforms in continuous mode for a
3V output application near VIN(MAX) = 33V.
As the input voltage is increased, the part is required
to switch for shorter periods of time. Delays associated
with turning off the power switch dictate the minimum
on time of the part. The minimum on time for the LT3493
is ~120ns. Figure 2 illustrates the switching waveforms
when the input voltage is increased to VIN = 35V.
VSW
20V/DIV
IL
0.5A/DIV
VOUT
200mV/DIV
AC COUPLED
COUT = 10μF
VOUT = 3V
VIN = 30V
ILOAD = 0.75A
L = 10μH
2μs/DIV
Figure 1
3493 F01
VSW
20V/DIV
IL
0.5A/DIV
VOUT
200mV/DIV
AC COUPLED
COUT = 10μF
VOUT = 3V
VIN = 35V
ILOAD = 0.75A
L = 10μH
2μs/DIV
Figure 2
3493 F02
3493fb
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