參數(shù)資料
型號(hào)： LT3508IUF#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24
封裝： 4 X 4 MM, PLASTIC, MO-220WGGD-X, QFN-24
文件頁(yè)數(shù)： 24/24頁(yè)
文件大?。?/td> 365K
代理商： LT3508IUF#TR
LT3508
9
3508f
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:
RR
V
OUT
12
08
1
=
.
–
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 VIN1 and VIN2 are tied together, the undervoltage
lockout is at 3.7V or below. If the two inputs are used
separately, then VIN1 has an undervoltage lockout of 3.7V
or below and VIN2 has an undervoltage lockout of 3V or
below. Because the internal supply runs off VIN1, chan-
nel 2 will not operate unless VIN1 > 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:
DC
VV
V
OUT
F
IN
SW
F
=
+
–
Unlike many xed 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 sufcient 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:
DCMAX
SW
=
+
1
β
where
βSW 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
VV
DC
VV
IN MIN
OUT
F
MAX
FSW
()
–
=
+
where VF is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.4V at maximum load).
Example: ISW = 1.5A and IBOOST = 50mA, VOUT = 3.3V,
βSW = 1.5A/50mA = 30, DCMAX = 1/(1+1/30) = 96%:
V
VV
V
IN MIN
()
..
%
–.
.
=
+
+=
33
04
96
04
38
Maximum Operating Voltage
The maximum operating voltage is determined by the
Absolute Maximum Ratings of the VIN and BOOST pins,
and by the minimum duty cycle:
DCMIN = tON(MIN) f
where tON(MIN) is equal to 130ns 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 volt-
age and the minimum duty cycle:
V
VV
DC
VV
IN PS
OUT
F
MIN
FSW
()
–
=
+
Example: f = 790kHz, VOUT = 3.3V, DCMIN = 130ns
790kHz = 0.103:
V
VV
V
IN PS
()
..
.
–.
.
=
+
+=
33
04
0 103
04
36
The LT3508 will regulate the output current at input voltages
greater than VIN(PS). For example, an application with an
output voltage of 1.8V and switching frequency of 1.5MHz
has a VIN(PS) of 11.3V, as shown in Figure 2. Figure 3
shows operation at 18V. Output ripple and peak inductor
current have signicantly increased. Exceeding VIN(PS) is
safe if the output is in regulation, if the external compo-
nents 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 VIN(PS) during start-up or overload conditions.
For operation above 20V in pulse skipping mode, program
the switching frequency to 1.1MHz or less.
相關(guān)PDF資料
PDF描述
LT3508IUF 3.2 A DUAL SWITCHING CONTROLLER, 1060 kHz SWITCHING FREQ-MAX, PQCC24
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