ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 21

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
For applications where the main input power is below 5V,
tie the V
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 7 to minimize the voltage drop caused by the
gate charge current. This will override the INTV
regulator and will prevent INTV
due to the dropout voltage. Make sure the INTV
is at or exceeds the R
which is typically 4.5V for logic level devices.
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
external diode DB from INTV
When one of the topside MOSFETs is to be turned on,
the driver places the C
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
MOSFET on, the boost voltage is above the input supply:
V
C
citance of the topside MOSFET(s). The reverse break-
down of the external Schottky diode must be greater than
V
at hot temperature to prevent leakage current feeding
INTV
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
BOOST
B
IN(MAX)
needs to be 100 times that of the total input capa-
CC
. When adjusting the gate drive level, the final arbiter
= V
IN
. Make sure the diode is a low leakage diode even
IN
and INTV
IN
B
and the BOOST pin follows. With the topside
in the Functional Diagram is charged though
+ V
LTC3869
Figure 7. Setup for a 5V Input
INTVCC
INTV
CC
V
DS(ON)
CC
IN
pins together and tie the combined
B
. The value of the boost capacitor
voltage across the gate source
CINTV
test voltage for the MOSFET
4.7µF
CC
B
CC
CC
when the SW pin is low.
connected to the BOOST
from dropping too low
R
B
1Ω
VIN
, DB)
+
C
3869 F07
5V
IN
CC
CC
voltage
linear
UVLO comparator constantly monitors the INTV
to ensure that an adequate gate-drive voltage is present. It
locks out the switching action when INTV
To prevent oscillation when there is a disturbance on the
INTV
hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
precision turn-on reference of 1.2V, one can use a resistor
divider to V
An extra 4.5µA 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 V
detection, V
C
The selection of C
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can be
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (V
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-
phase technique typically reduces the input capacitor’s RMS
ripple current by a factor of 30% to 70% when compared
to a single phase power supply solution.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
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:
This formula has a maximum at V
I
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.
OUT
IN
C
and C
/2. This simple worst-case condition is commonly used
IN
CC
, the UVLO comparator has 600mV of precision
Required I
OUT
IN
IN
to turn on the IC when V
Selection
OUT
IN
needs to be higher than 4.5V.
LTC3869/LTC3869-2
supply. Because the RUN pins have a
)(I
IN
RMS
is simplified by the 2-phase architec-
OUT
≈
) product needs to be used in the
I
MAX
V
IN
⎡ ⎣
(
V
IN
OUT
OUT
= 2V
)
)/(V
IN
(
OUT
V
CC
IN
IN
is high enough.
IN
, where I
is below 3.2V.
undervoltage
– V
). To prevent
OUT
CC
21
voltage
)
RMS
⎤ ⎦
1/ 2
3869f
=