LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 10

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3502/LT3502A
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 the LT3502/LT3502A applica-
tions depends on the output voltage and on the absolute
maximum ratings of the V
The minimum input voltage is determined by either the
LT3502/LT3502A’s minimum operating voltage of 3V, 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
(~0.4V) and V
(~0.45V at maximum load). This leads to a minimum input
voltage of:
with DC
LT3502.
The maximum input voltage is determined by the
absolute maximum ratings of the V
fi xed frequency operation, the maximum input voltage is
determined by the minimum duty cycle DC
DC
10
R1= R2
DC =
MIN
V
V
IN(MIN)
IN(MAX)
= 0.15 for the LT3502A and 0.08 for the LT3502.
D
MAX
V
IN
is the forward voltage drop of the catch diode
V
=
V
0.8V
=
– V
OUT
= 0.80 for the LT3502A and 0.90 for the
OUT
SW
V
V
OUT
DC
OUT
SW
DC
+ V
is the voltage drop of the internal switch
– 1
MAX
MIN
+ V
+ V
+ V
D
D
D
D
– V
– V
IN
D
and BOOST pins.
D
+ V
+ V
SW
IN
SW
and BOOST pins. For
MIN
:
Note that this is a restriction on the operating input volt-
age for fi xed frequency operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings of
the V
limited to the V
conditions.
Minimum On Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed V
output voltage ripple increases as the input voltage is
increased.
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 LT3502/LT3502A
is 60ns (Figure 1).
When the required on time decreases below the minimum
on time of 60ns, instead of the switch pulse width becoming
narrower to accommodate the lower duty cycle require-
ment, the switch pulse width remains fi xed at 60ns. The
inductor current ramps up to a value exceeding the load cur-
rent and the output ripple increases. The part then remains
500mA/DIV
100mV/DIV
20V/DIV
IN
V
V
OUT
and BOOST pins. The input voltage should be
SW
I
Figure 1. Continuous Mode Operation Near
Minimum On Time of 60ns
L
V
V
L = 6.8μH
C
I
OUT
IN
OUT
OUT
= 33V
= 250mA
= 3.3V
= 10μF
IN
operating range (40V) during overload
IN(MAX)
1μs/DIV
(up to 40V), however, the
3502 F01
3502fc