參數(shù)資料
型號(hào)： LTC3775EMSE#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO16
封裝： LEAD FREE, PLASTIC, MSOP-16
文件頁(yè)數(shù)： 4/34頁(yè)
文件大?。?/td> 534K
代理商： LTC3775EMSE#TRPBF
LTC3775
12
3775fa
APPLICATIONS INFORMATION
Output Overvoltage Protection
An overvoltage comparator, MAX, guards against transient
overshoots (>10%) as well as other more serious condi-
tions that may overvoltage the output. In such cases, the
top MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Run/Shutdown
The LTC3775 can be put into a low power shutdown mode
with quiescent current <14μA by pulling the RUN/SHDN
pin below 0.74V. The RUN/SHDN pin can also be used as
an accurate external UVLO (undervoltage lockout) input
with a threshold of 1.22V. The driver outputs stay low if
this pin is <1.22V. The external resistive divider R4 and R5
shown in the Block Diagram can be used to set the UVLO
level based on VIN. The VIN voltage at which the switching
starts is given by the following formula:
UVLO (Upper) = 1.22V (1 + R4/R5) – (1μA R4)
The RUN/SHDN pin has an internal 1μA pull-up for default
turn-on if this pin is left oating. This 1μA pull-up current is
included in the above UVLO calculation. When RUN/SHDN
goes above 1.22V, this pull-up current is increased to 5μA.
This provides some amount of hysteresis to the UVLO
threshold. The lower UVLO level becomes:
UVLO (Lower) = 1.22V (1 + R4/R5) – (5μA R4)
So the amount of hysteresis is given by:
UVLO (Hysteresis) = 4μA R4
Soft-Start
The LTC3775 includes a soft-start circuit that provides a
smooth output voltage ramp during start-up. The SS pin
requires an external capacitor, CSS, to GND with the value
determined by the required soft-start time. An internal 1μA
current source charges CSS. When the voltage on the SS
pin is less than the 0.6V internal reference, the LTC3775
regulates the VFB voltage to the SS pin voltage instead of
the 0.6V reference. As the SS voltage rises linearly from
0V to 0.6V and beyond, the output voltage, VOUT, rises
smoothly from zero to its nal value. The total soft-start
time can be calculated as:
t
C
μA
SOFTSTART
SS
=
09
1
.
The SS pin is pulled low in the following conditions: during
an LDO undervoltage condition (INTVCC < 3.6V), during
shutdown (RUN pin < 1.22V), during an overtemperature
condition (TJ > 165°C) and during current limit.
If either the top or bottom current limit comparator trips,
the SS pin is pulled low until the inductor current regu-
lates at around the current limit setting. Once the fault is
cleared, SS will start charging up allowing the duty cycle
and output voltage to increase gradually. Due to the cur-
rent limit action on the SS pin, it is important to avoid
an overcurrent condition during start-up of the power
supply, or VOUT will fail to start up properly.
0.74V
+
–
+
–
1.22V
TURN OFF TG
EN
4μA
RUN
R4
R5
3775 F02
IRUN
1μA
VIN
SHDN
LTC3775
CHIP
SHUTDOWN
Figure 2. RUN Pin Control
Figure 3. Typical Start-Up Waveform
for a Buck Converter Using the LTC3775
IL
5A/DIV
VOUT
0.5V/DIV
VSS
1V/DIV
2ms/DIV
3775 F03
VIN = 12V
VOUT = 1.2V
CSS = 0.01μF
MODE = 0V
SW FREQ = 500kHz
SWITCHOVER
FROM PULSE-
SKIPPING TO
CONTINUOUS
MODE
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