LT3502 Datasheet, PDF (10/24 Pages) Linear Technology – 1.1MHz/2.2MHz, 500mA Step-Down Regulators in 2mm × 2mm DFN and MS10

LT3502/LT3502A
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
^_0V^O_.8^U_V^T – 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.
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 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 fixed at 60ns. The
inductor current ramps up to a value exceeding the load cur-
rent and the output ripple increases. The part then remains
V_SW
20V/DIV
I_L
500mA/DIV
V_OUT
100mV/DIV
V_IN = 33V
V_OUT = 3.3V
L = 6.8μH
C_OUT = 10μF
I_OUT = 250mA
1μs/DIV
3502 F01
Figure 1. Continuous Mode Operation Near
Minimum On Time of 60ns
3502fc
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