LT3508
APPLICATIONS INFORMATION
Setting the Output Voltage
where V is the forward voltage drop of the catch diode
F
(~0.4V) and V is the voltage drop of the internal switch
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
(~0.4V at maximum load).
Example: I = 1.5A and I
SW
= 50mA, V
= 3.3V,
OUT
SW
BOOST
β
= 1.5A/50mA = 30, DC
= 1/(1+1/30) = 96%:
MAX
V
0.8V
⎛
⎞
⎠
OUT
R1= R2
– 1
⎜
⎝
⎟
3.3V + 0.4V
V
=
– 0.4V + 0.4V = 3.8V
IN(MIN)
96%
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Maximum Operating Voltage
Minimum Operating Voltage
The maximum operating voltage is determined by the
Absolute Maximum Ratings of the V and BOOST pins,
IN
The minimum operating voltage is determined either by
theLT3508’sundervoltagelockoutorbyitsmaximumduty
cycle. If V and V are tied together, the undervoltage
and by the minimum duty cycle:
DC
= t
• f
IN1
IN2
MIN
ON(MIN)
lockout is at 3.7V or below. If the two inputs are used
separately, then V has an undervoltage lockout of 3.7V
where t
is equal to 130ns (for T > 125°C t
J ON(MIN)
ON(MIN)
IN1
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:
or below and V has an undervoltage lockout of 3V or
IN2
below. Because the internal supply runs off V , chan-
IN1
nel 2 will not operate unless V > 3.7V. The duty cycle
IN1
is the fraction of time that the internal switch is on and is
determined by the input and output voltages:
VOUT + VF
DCMIN
VOUT + VF
DC =
V
=
– VF + VSW
IN(PS)
V – VSW + VF
IN
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-
tionandforcestheswitchtoturnoff, allowingtheinductor
current to charge up the boost capacitor. This places a
limitation on the maximum duty cycle as follows:
Example:f=790kHz, V
= 0.103:
=3.3V, DC =130ns•790kHz
OUT
MIN
3.3V + 0.4V
V
=
– 0.4V + 0.4V = 36V
IN(PS)
0.103
TheLT3508willregulatetheoutputcurrentatinputvoltages
greater than V . For example, an application with an
IN(PS)
output voltage of 1.8V and switching frequency of 1.5MHz
hasaV of11.3V,asshowninFigure2.Figure3shows
IN(PS)
operation at 18V. Output ripple and peak inductor current
have significantly increased. Exceeding V is safe if
theoutputisinregulation,iftheexternalcomponentshave
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
1
DCMAX
=
1
IN(PS)
1+
βSW
where β is equal to the SW pin current divided by the
SW
BOOST pin current as shown in the Typical Performance
Characteristics section. This leads to a minimum input
voltage of:
V
duringstart-uporoverloadconditions(foroutputs
IN(PS)
greater than 5V, use V
= 5V to calculate V
). For
OUT
IN(PS)
operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
VOUT + VF
DCMAX
V
=
– VF + VSW
IN(MIN)
3508fb
9
Linear [ Linear ]
