LT3508
APPLICATIONS INFORMATION
Setting the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
⎛
V
⎞
R1
=
R2
⎜
OUT
– 1
⎝
0.8 V
⎟
⎠
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Minimum Operating Voltage
The minimum operating voltage is determined either by
the LT3508’s undervoltage lockout or by its maximum duty
cycle. If V
IN1
and V
IN2
are tied together, the undervoltage
lockout is at 3.7V or below. If the two inputs are used
separately, then V
IN1
has an undervoltage lockout of 3.7V
or below and V
IN2
has an undervoltage lockout of 3V or
below. Because the internal supply runs off V
IN1
, chan-
nel 2 will not operate unless V
IN1
> 3.7V. The duty cycle
is the fraction of time that the internal switch is on and is
determined by the input and output voltages:
V
OUT
+
V
F
DC
=
V
IN
– V
SW
+
V
F
Unlike many fixed frequency regulators, the LT3508 can
extend its duty cycle by turning on for multiple cycles. The
LT3508 will not switch off at the end of each clock cycle if
there is sufficient voltage across the boost capacitor (C3
in Figure 1). Eventually, the voltage on the boost capacitor
falls and requires refreshing. Circuitry detects this condi-
tion and forces the switch to turn off, allowing the inductor
current to charge up the boost capacitor. This places a
limitation on the maximum duty cycle as follows:
DC
MAX
=
1
1
+
1
β
SW
where V
F
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.4V at maximum load).
Example: I
SW
= 1.5A and I
BOOST
= 50mA, V
OUT
= 3.3V,
β
SW
= 1.5A/50mA = 30, DC
MAX
= 1/(1+1/30) = 96%:
V
IN(MIN)
=
3.3V
+
0.4V
– 0.4V
+
0.4V
=
3.8 V
96%
Maximum Operating Voltage
The maximum operating voltage is determined by the
Absolute Maximum Ratings of the V
IN
and BOOST pins,
and by the minimum duty cycle:
DC
MIN
= t
ON(MIN)
• f
where t
ON(MIN)
is equal to 130ns (for T
J
> 125°C t
ON(MIN)
is equal to 150ns) and f is the switching frequency.
Running at a lower switching frequency allows a lower
minimum duty cycle. The maximum input voltage before
pulse skipping occurs depends on the output voltage and
the minimum duty cycle:
V
IN(PS)
=
V
OUT
+
V
F
– V
F
+
V
SW
DC
MIN
Example: f = 790kHz, V
OUT
= 3.3V, DC
MIN
= 130ns • 790kHz
= 0.103:
V
IN(PS)
=
3.3V
+
0.4V
– 0.4V
+
0.4V
=
36 V
0.103
where
β
SW
is equal to the SW pin current divided by the
BOOST pin current as shown in the Typical Performance
Characteristics section. This leads to a minimum input
voltage of:
V
IN(MIN)
=
V
OUT
+
V
F
– V
F
+
V
SW
DC
MAX
The LT3508 will regulate the output current at input voltages
greater than V
IN(PS)
. For example, an application with an
output voltage of 1.8V and switching frequency of 1.5MHz
has a V
IN(PS)
of 11.3V, as shown in Figure 2. Figure 3 shows
operation at 18V. Output ripple and peak inductor current
have significantly increased. Exceeding V
IN(PS)
is safe if
the output is in regulation, if the external components have
adequate ratings to handle the peak conditions and if the
peak inductor current does not exceed 3.2A. A saturating
inductor may further reduce performance. Do not exceed
V
IN(PS)
during start-up or overload conditions (for outputs
greater than 5V, use V
OUT
= 5V to calculate V
IN(PS)
). For
operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
3508fb
9
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