ltc3862gn-1 Linear Technology Corporation, ltc3862gn-1 Datasheet - Page 15

• Manufacturer Part Number: ltc3862gn-1
• Description: Multi-phase Current Mode Step-up Dc/dc Controller
• Manufacturer: Linear Technology Corporation

Operation at High Supply Voltage
At high input voltages, the LTC3862-1’s internal LDO
can dissipate a signifi cant amount of power, which could
cause the maximum junction temperature to be exceeded.
Conditions such as a high operating frequency, or the use
of more than one power MOSFET per channel, could push
the junction temperature rise to high levels. If the thermal
equations above indicate too high a rise in the junction
temperature, an external bias supply can always be used
to reduce the power dissipation on the IC, as shown in
Figure 3.
For example, a 12V system rail that is available would be
more suitable than the 24V main input power rail to power
the LTC3862-1. Also, the bias power can be generated with
a separate switching or LDO regulator. An example of an
LDO regulator is shown in Figure 3. The output voltage
of the LDO regulator can be set by selecting an appropri-
ate zener diode to be higher than 10V but low enough to
divide the power dissipation between LTC3862-1 and Q1
in Figure 3. The absolute maximum voltage rating of the
INTV
Power Supply Sequencing
As shown in Figure 1, there are body diodes in parallel with
the PMOS output transistors in the two LDO regulators
in the LTC3862-1. As a result, it is not possible to bias
the INTV
OPERATION
Figure 3. Using the LTC3862-1 with an External Bias Supply
CC
pin is 11V.
CC
and V
R1
IN
D1
pins of the chip from separate power
V
IN
Q1
C
VCC
V
INTV
IN
LTC3862-1
CC
38621 F03
supplies. Independently biasing the INTV
separate power supply can cause one of two possible
failure modes during supply sequencing. If the INTV
supply comes up before the V
fl ow from the external INTV
diode of the LDO PMOS device, to the input capacitor
and V
condition and cause catastrophic failure of the IC.
If, however, the V
INTV
load to the internal LDO in the LTC3862-1, and the LDO will
attempt to charge the INTV
current. This will result in excessive power dissipation and
possible thermal overload of the LTC3862-1.
Programming the Output Voltage
The output voltage is set by a resistor divider according
to the following formula:
The external resistor divider is connected to the output
as shown in Figure 4. Resistor R1 is normally chosen so
that the output voltage error caused by the current fl owing
out of the V
compared to the current in the divider. For an output volt-
age error due to the error amp input bias current of less
than 0.5%, this translates to a maximum value of R1 of
about 30k.
V
OUT
CC
IN
pin. This high current fl ow could trigger a latchup
supply, the external INTV
= 1.223V 1+
Figure 4. Programming the Output Voltage
with a Resistor Divider
FB
pin during normal operation is negligible
IN
LTC3862-1
supply to the IC comes up before the
SGND
R2
R1
FB
CC
CC
output with its short-circuit
IN
supply, through the body
V
supply, high current will
OUT
CC
R2
R1
38621 F04
LTC3862-1
supply will act as a
CC
pin from a
15
38621f
CC