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參數(shù)資料
| 型號： | LTC3835IUFD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封裝： | 4 X 5 MM, PLASTIC, MO-220, QFN-20 |
| 文件頁數(shù)： | 10/30頁 |
| 文件大?。?/td> | 1775K |
| 代理商： | LTC3835IUFD |
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LTC3835
18
3835fd
APPLICATIONS INFORMATION
4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 6.
Fault Conditions: Current Limit and Current Foldback
The LTC3835 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 70% of its nominal output level, then the
maximum sense voltage is progressively lowered from
100mV to 30mV. Under short-circuit conditions with very
low duty cycles, the LTC3835 will begin cycle skipping 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 cur-
rent is determined by the minimum on-time tON(MIN) of the
LTC3835 (≈180ns), the input voltage and inductor value:
IL(SC) = tON(MIN) (VIN/L)
The resulting short-circuit current is:
ISC =
10mV
RSENSE
–
1
2
IL(SC)
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
muchhigherthannominallevels.Thecrowbarcauseshuge
currents to flow, that blow the fuse to protect against a
shortedtopMOSFETiftheshortoccurswhilethecontroller
is operating.
A comparator monitors the output for overvoltage
conditions.Thecomparator(OV)detectsovervoltagefaults
greater than 10% above the nominal output voltage. When
this condition is sensed, the top MOSFET is turned off and
the bottom MOSFET is turned on until the overvoltage
condition is cleared. The bottom MOSFET remains on
continuously for as long as the overvoltage condition
persists; if VOUT returns to a safe level, normal operation
automaticallyresumes.AshortedtopMOSFETwillresultin
a high current condition which will open the system fuse.
The switching regulator will regulate properly with a leaky
top MOSFET by altering the duty cycle to accommodate
the leakage.
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitors CB connected to the BOOST
pinssupplythegatedrivevoltagesforthetopsideMOSFET.
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 places the CB voltage across the gate-source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to VIN and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
VBOOST = VIN + VINTVCC. The value of the boost capacitor
CBneedstobe100timesthatofthetotalinputcapacitance
of the topside MOSFET. 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.
Figure 6. Capacitive Charge Pump for EXTVCC
EXTVCC
VIN
TG1
SW
BG1
PGND
LTC3835
RSENSE
VOUT
VN2222LL
+
COUT
3835 F06
N-CH
+
CIN
1F
VIN
L1
BAT85
0.22F
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