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參數(shù)資料
| 型號： | LT3502IDC#TRM |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.1 A SWITCHING REGULATOR, 1400 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | 2 X 2 MM, PLASTIC, DFN-10 |
| 文件頁數(shù)： | 19/20頁 |
| 文件大?。?/td> | 457K |
| 代理商： | LT3502IDC#TRM |
LT3502/LT3502A
8
3502f
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:
RR
V
OUT
12
08
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 VIN 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
VV
V
OUT
D
IN
SW
D
=
+
–
where VD is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.45V at maximum load). This leads to a minimum input
voltage of:
V
VV
DC
VV
IN MIN
OUT
D
MAX
DSW
()
–
=
+
with DCMAX = 0.80 for the LT3502A and 0.90 for the
LT3502.
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins. For xed
frequency operation, the maximum input voltage is de-
termined by the minimum duty cycle DCMIN:
V
VV
DC
VV
IN MAX
OUT
D
MIN
DSW
()
–
=
+
DCMIN = 0.10 for the LT3502A and 0.05 for the LT3502.
Note that this is a restriction on the operating input volt-
age for xed frequency operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings of
the VIN and BOOST pins. The input voltage should be
limited to the VIN operating range (40V) during overload
conditions.
Minimum On Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed VIN(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 xed at 60ns. The
inductor current ramps up to a value exceeding the load cur-
rent and the output ripple increases. The part 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 VIN(MAX) is safe and will not
damage the part.
As the input voltage increases, the inductor current ramps
up quicker, the number of skipped pulses increases and
Figure 1. Continuous Mode Operation Near
Minimum On Time of 60ns.
VSW
20V/DIV
VOUT
100mV/DIV
1
μs/DIV
VIN = 33V
VOUT = 3.3V
L = 6.8
μH
COUT = 10μF
IOUT = 250mA
3502 F01
IL
500mA/DIV
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