Datasheet

LTC3869/LTC3869-2
22
38692fa
For more information www.linear.com/LTC3869
applicaTions inForMaTion
For applications where the main input power is below 5V,
tie the V
IN
and INTV
CC
pins together and tie the combined
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
CC
linear
regulator and will prevent INTV
CC
from dropping too low
due to the dropout voltage. Make sure the INTV
CC
voltage
is at or exceeds the R
DS(ON)
test voltage for the MOSFET
which is typically 4.5V for logic level devices.
UVLO comparator constantly monitors the INTV
CC
voltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTV
CC
is below
3.2V. To prevent oscillation when there is a disturbance
on the INTV
CC
, the UVLO comparator has 600mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pins have a
precision turn-on reference of 1.2V, one can use a resistor
divider to V
IN
to turn on the IC when V
IN
is high enough.
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
IN
undervoltage
detection, V
IN
needs to be higher than 4.5V.
C
IN
and C
OUT
Selection
The selection of C
IN
is simplified by the 2-phase architec-
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 oc-
curs when only one controller is operating. The controller
with the highest (V
OUT
)(I
OUT
) product needs to be used
in the formula below to determine the maximum RMS
capacitor current requirement. Increasing the output cur-
rent 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
OUT
)/(V
IN
). 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:
C
IN
Required I
RMS
≈
I
MAX
V
IN
V
OUT
( )
V
IN
– V
OUT
( )
⎡
⎣
⎤
⎦
1/2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst-case condition is commonly used
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 (C
B
, DB)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
B
in the Functional Diagram is charged
though external diode DB from INTV
CC
when the SW pin
is low. When one of the topside MOSFETs is to be turned
on, the driver places the C
B
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 V
IN
and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
. The value of the boost capacitor
C
B
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
V
IN(MAX)
. Make sure the diode is a low leakage diode even
at hot temperature to prevent leakage current feeding
INTV
CC
. 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.
Undervoltage Lockout
The LTC3869 has two functions that help protect the
controller in case of undervoltage conditions. A precision
INTV
CC
LTC3869
R
VIN
1Ω
C
IN
3869 F07
4.7µF
5V
CINTV
CC
+
V
IN