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参数资料
| 型号： | LTC3812IFE-5#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, PDSO16 |
| 封装： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件页数： | 13/32页 |
| 文件大小： | 425K |
| 代理商： | LTC3812IFE-5#TR |
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LTC3812-5
20
38125fb
the drivers are turned off to prevent the SOT-23 MOSFET
from overheating. Soft-start cycles are then attempted at
low duty cycle intervals to try to bring the output back
up (see Figure 9). This fault timeout operation is enabled
by choosing the choosing RNDRV such that the resistor
current INDRV is greater than 270μA by using the follow-
ing formulas:
RNDRV
PMOSFET(MAX)/ICC VT
270
μA
where
ICC = (f)(QG(TOP) + QG(BOTTOM)) + 3mA
and VT is the threshold voltage of the MOSFET.
The value of RNDRV also affects the VIN(MIN) as follows:
VIN(MIN) = VINTVCC(MIN) + (40μA) RNDRV + VT
(1)
where VINTVCC(MIN) is normally 4.5V for driving logic level
MOSFETs. If minimum VIN is not low enough, consider
reducing RNDRV and/or using a darlington NPN instead of
an NMOS to reduce VT to ~1.4V.
When using RNDRV equal to the computed value, the
LTC3812-5 will enable the low duty cycle soft-start re-
tries only when the desired maximum power dissipation,
PMOSFET(MAX), in the MOSFET is exceeded and leave the
drivers on continuously otherwise. The shutoff/restart
times are a function of the RUN/SS capacitor value.
The external NMOS for the linear regulator should be a
standard 3V threshold type (i.e., not a logic level threshold).
The rate of charge of VCC from 0V to 5.5V is controlled
by the LTC3812-5 to be approximately 75μs regardless of
the size of the capacitor connected to the INTVCC pin. The
charging current for this capacitor is approximately:
IC =
5.5V
75
μs
CINTVCC
The safe operating area (SOA) for the external NMOS
should be chosen so that capacitor charging does not
damage the NMOS. Excessive values of capacitor are
unnecessary and should be avoided. Typically values in
the 1μF to 10μF work well.
One more design requirement for this mode is the minimum
soft-start capacitor value. The fault timeout is enabled
when RUN/SS voltage is greater than 4V. This gives the
power supply time to bring the output up before it starts
the timeout sequence. To prevent timeout sequence from
starting prematurely during start-up, a minimum CSS value
is necessary to ensure that VRUN/SS < 4V until VEXTVCC >
4.7V. To ensure this, choose:
CSS > COUT (2.3 10-6)/IOUT(MAX)
Mode 2 should be used if VOUT is outside of the 4.7V <
EXTVCC < 15V operating range and the extra complexity
of a charge pump or extra inductor winding is not wanted
Figure 9. Fault Timeout Operation
APPLICATIONS INFORMATION
RUN/SS
VOUT
TG/BG
FAULT TIMEOUT
ENABLED
EXTVCC UV THRESHOLD
DRIVER OFF THRESHOLD
DRIVER POWER
FROM VIN
SHORT-CIRCUIT EVENT
START-UP INTO SHORT CIRCUIT
START-UP
DRIVER POWER
FROM VIN
ISS/TRACK = 1.4μA (SOURCE)
ISS/TRACK = 0.1μA (SINK)
38125 F09
DRIVER POWER
FROM VOUT
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