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參數(shù)資料
| 型號： | LTC3867IUF#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.1 A SWITCHING CONTROLLER, 1200 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封裝： | 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24 |
| 文件頁數(shù)： | 20/36頁 |
| 文件大?。?/td> | 380K |
| 代理商： | LTC3867IUF#PBF |
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LTC3867
27
3867f
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitor, CB, connected to the BOOST
pinsuppliesthegatedrivevoltagesforthetopsideMOSFET.
Capacitor CB in the Functional Diagram is charged though
external diode DB from INTVCC when the SW pin is low.
When the topside MOSFET is to be turned on, the driver
placestheCBvoltageacrossthegatesourceoftheMOSFET.
ThisenhancestheMOSFETandturnsonthetopsideswitch.
The switch node voltage, SW, rises to VIN and the BOOST
pin follows. With the topside MOSFET on, the boost volt-
age is above the input supply:
VBOOST = VIN + VINTVCC – VDB
The value of the boost capacitor, CB, needs to be 100 times
thatofthetotalinputcapacitanceofthetopsideMOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than VIN(MAX). When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Setting Output Voltage
The LTC3867 output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 13. The regulated output voltage is
determined by:
VOUT = 0.6V 1+
RB
RA
To improve the frequency response, a feedforward ca-
pacitor, CFF, may be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
If the diffamp is used, then the resistor divider center tap
should connect to the noninverting input of the diffamp,
DIFF+. DIFFOUT should then be shorted to VFB.
Fault Conditions: Current Limit and Current Foldback
The LTC3867 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put falls below 50% of its nominal output level, then the
maximum sense voltage is progressively lowered from its
maximumprogrammedvaluetoone-thirdofthemaximum
value.Foldbackcurrentlimitingisdisabledduringthesoft-
start or tracking up using the TK/SS pin. It is not disabled
for internal soft-start. Under short-circuit conditions with
verylowdutycycles,theLTC3867willbegincycleskipping
in order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short circuit ripple
current is determined by the minimum on-time tON(MIN) of
theLTC3867(≈65ns),theinputvoltageandinductorvalue:
IL(SC) = tON(MIN)
VIN
L
The resulting short-circuit current is:
ISC =
1/3 VSENSE(MAX)
RSENSE
–
1
2
IL SC
( )
Afterashort,orwhilestartingwithinternalsoft-start,make
sure that the load current takes the folded-back current
limit into account.
Figure 12. Setup for a 5V Input
Figure 13. Setting Output Voltage
APPLICATIONS INFORMATION
RVIN
1
CIN
3867 F12
5V
CINTVCC
4.7F
+
INTVCC
LTC3867
VIN
LTC3867
VFB
VOUT
RB
CFF
RA
3867 F13
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