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参数资料
| 型号： | LTC3869IGN-2#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO28 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28 |
| 文件页数： | 14/40页 |
| 文件大小： | 2705K |
| 代理商： | LTC3869IGN-2#TRPBF |
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LTC3869/LTC3869-2
21
3869f
APPLICATIONS INFORMATION
For applications where the main input power is below 5V,
tie the VIN and INTVCC pins together and tie the combined
pins to the 5V input with a 1 or 2.2 resistor as shown
in Figure 7 to minimize the voltage drop caused by the
gate charge current. This will override the INTVCC linear
regulator and will prevent INTVCC from dropping too low
due to the dropout voltage. Make sure the INTVCC voltage
is at or exceeds the RDS(ON) test voltage for the MOSFET
which is typically 4.5V for logic level devices.
UVLO comparator constantly monitors the INTVCCvoltage
to ensure that an adequate gate-drive voltage is present. It
locks out the switching action when INTVCC is below 3.2V.
To prevent oscillation when there is a disturbance on the
INTVCC, the UVLO comparator has 600mV of precision
hysteresis.
Another way to detect an undervoltage condition is to
monitor the VIN supply. Because the RUN pins have a
precision turn-on reference of 1.2V, one can use a resistor
divider to VIN to turn on the IC when VIN is high enough.
An extra 4.5A of current flows out of the RUN pin once
the RUN pin voltage passes 1.2V. One can program the
hysteresis of the run comparator by adjusting the values
of the resistive divider. For accurate VIN undervoltage
detection, VIN needs to be higher than 4.5V.
CIN and COUT Selection
The selection of CIN is simplified by the 2-phase architec-
ture and its impact on the worst-case RMS current drawn
throughtheinputnetwork(battery/fuse/capacitor).Itcanbe
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (VOUT)(IOUT) product needs to be used in the
formula below to determine the maximum RMS capacitor
current requirement. Increasing the output current drawn
from the other controller will actually decrease the input
RMS ripple current from its maximum value. The out-of-
phasetechniquetypicallyreducestheinputcapacitor’sRMS
ripple current by a factor of 30% to 70% when compared
to a single phase power supply solution.
Incontinuousmode,thesourcecurrentofthetopMOSFET
is a square wave of duty cycle (VOUT)/(VIN). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
CIN Required IRMS ≈
IMAX
VIN
VOUT
(
) VIN – VOUT
(
)
1/2
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT/2.Thissimpleworst-caseconditioniscommonlyused
for design because even significant deviations do not of-
fer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
Figure 7. Setup for a 5V Input
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitors CB connected to the BOOST
pinssupplythegatedrivevoltagesforthetopsideMOSFETs.
Capacitor CB in the Functional Diagram is charged though
external diode DB from INTVCC when the SW pin is low.
When one of the topside MOSFETs 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
CB needs to be 100 times that of the total input capa-
citance of the topside MOSFET(s). The reverse break-
down of the external Schottky diode must be greater than
VIN(MAX). Make sure the diode is a low leakage diode even
at hot temperature to prevent leakage current feeding
INTVCC.Whenadjustingthegatedrivelevel,thefinalarbiter
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.
Undervoltage Lockout
The LTC3869 has two functions that help protect the
controller in case of undervoltage conditions. A precision
INTVCC
LTC3869
RVIN
1
CIN
3869 F07
4.7F
5V
CINTVCC
+
VIN
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