Datasheet

LT3502/LT3502A
10
3502fd
applications inForMation
Figure 1. Continuous Mode Operation Near
Minimum On-Time of 60ns
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
0.8V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
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
IN
and BOOST pins.
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:
DC =
V
OUT
+
V
D
V
IN
– V
SW
+ V
D
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.45V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
=
V
OUT
+
V
D
DC
MAX
– V
D
+ V
SW
with DC
MAX
= 0.80 for the LT3502A and 0.90 for the
LT3502.
The maximum input voltage is determined by the
absolute maximum ratings of the V
IN
and BOOST pins. For
fixed frequency operation, the maximum input voltage is
determined by the minimum duty cycle DC
MIN
:
V
IN(MAX)
=
V
OUT
+
V
D
DC
MIN
– V
D
+ V
SW
DC
MIN
= 0.15 for the LT3502A and 0.08 for the LT3502.
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 33V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 250mA
3502 F01
I
L
500mA/DIV
Note that this is a restriction on the operating input volt-
age for fixed frequency operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings of
the V
IN
and BOOST pins. The input voltage should be
limited to the V
IN
operating range (40V) during overload
conditions.
Minimum On-Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed V
IN(MAX)
(up to 40V), however, the
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 mini
-
mum on-time of 60ns, instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the switch pulse width remains fixed at
60ns. The inductor current ramps up to a value exceed
-
ing the load current and the output ripple increases. The
pa
rt then remains off until the output voltage dips below
the programmed value before it begins switching again
(Figure 2).
Provided that the load can tolerate the increased output
voltage ripple and that the components have been properly
selected, operation above V
IN(MAX)
is safe and will not
damage the part.