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參數(shù)資料
| 型號： | LT8415IDDB#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.03 A SWITCHING REGULATOR, PDSO12 |
| 封裝： | 3 X 2 MM, LEAD FREE, PLASTIC, DFN-12 |
| 文件頁數(shù)： | 12/12頁 |
| 文件大?。?/td> | 182K |
| 代理商： | LT8415IDDB#PBF |
LT8415
9
8415f
APPLICATIONS INFORMATION
Maximum Output Load Current
The maximum output current of a particular LT8415 circuit
is a function of several circuit variables. The following
method can be helpful in predicting the maximum load
current for a given circuit:
Step 1: Calculate the peak inductor current:
IPK = ILIMIT +
VIN 150 106
L
mA
where ILIMIT is 25mA. L is the inductance value in Henrys
and VIN is the input voltage to the boost circuit.
Step 2: Calculate the inductor ripple current:
IRIPPLE =
(VOUT + 1 VIN) 200 106
L
mA
where VOUT is the desired output voltage. If the inductor
ripple current is less than the peak current, then the circuit
will only operate in discontinuous conduction mode. The
inductor value should be increased so that IRIPPLE < IPK.
An application circuit can be designed to operate only in
discontinuous mode, but the output current capability
will be reduced.
Step 3: Calculate the average input current:
IIN(AVG) = IPK
IRIPPLE
2
mA
Step 4: Calculate the nominal output current:
IOUT(NOM) =
IIN(AVG) VIN 0.7
VOUT
mA
Step 5: Derate output current:
IOUT = IOUT(NOM) 0.8
For low output voltages the output current capability will
be increased. When using output disconnect (load current
taken from VOUT), these higher currents will cause the
drop in the PMOS switch to be higher resulting in lower
output current capability than predicted by the preceding
equations.
Inrush Current
When VCC is stepped from ground to the operating voltage
while the output capacitor is discharged, a high level of
inrush current may ow through the inductor and Schottky
diode into the output capacitor. Conditions that increase
inrush current include a larger more abrupt voltage step
at VCC, a larger output capacitor tied to the CAP pin and
an inductor with a low saturation current. While the chip is
designed to handle such events, the inrush current should
not be allowed to exceed 0.3A. For circuits that use output
capacitor values within the recommended range and have
input voltages of less than 6V, inrush current remains low,
posing no hazard to the device. In cases where there are
large steps at VCC (more than 6V) and/or a large capacitor
is used at the CAP pin, inrush current should be measured
to ensure safe operation.
Soft-Start
The LT8415 contains a soft-start circuit to limit peak switch
currents during start-up. High start-up current is inherent
in switching regulators in general since the feedback loop
is saturated due to VOUT being far from its nal value. The
regulator tries to charge the output capacitor as quickly
as possible, which results in large peak current.
When the FBP pin voltage is generated by a resistor divider
from the VREF pin, the start-up current can be limited by
connecting an external capacitor (typically 47nF to 220nF)
to the VREF pin. When the part is brought out of shutdown,
this capacitor is rst discharged for about 70μs (providing
protection against pin glitches and slow ramping), then
an internal 10μA current source pulls the VREF pin slowly
to 1.235V. Since the VOUT voltage is set by the FBP pin
voltage, the VOUT voltage will also slowly increase to the
regulated voltage, which results in lower peak inductor
current. The voltage ramp rate on the pin can be set by
the value of the VREF pin capacitor.
Output Disconnect
The LT8415 has an output disconnect PMOS that blocks
the load from the input during shutdown. The maximum
current through the PMOS is limited to 19mA by circuitry
inside the chip, helping the chip survive output shorts.
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