LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 15

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

To prevent the maximum junction temperature from be-
ing exceeded, the input supply current to the IC should
be checked when operating in continuous mode (heavy
load) at maximum V
ing frequency and the size of the power MOSFETs may
need to be made in order to maintain a reliable junction
temperature. Finally, it is important to verify the calcula-
tions by performing a thermal analysis of the final PCB
using an infrared camera or thermal probe. As an option,
an external regulator shown in Figure 3 can be used to
reduce the total power dissipation on the IC.
Thermal Shutdown Protection
In the event of an overtemperature condition (external
or internal), an internal thermal monitor will shut down
the gate drivers and reset the soft-start capacitor if the
die temperature exceeds 170°C. This thermal sensor has
a hysteresis of 10°C to prevent erratic behavior at hot
temperatures. The LTC3862-2’s internal thermal sen-
sor is intended to protect the device during momentary
overtemperature conditions. Continuous operation above
the specified maximum operating junction temperature,
however, may result in device degradation.
Operation at Low Supply Voltage
The LTC3862-2 has a minimum input voltage of 5.5V,
making it a good choice for applications that require high
voltage power MOSFETs with 6V R
driver for the LTC3862-2 consists of PMOS pull-up and
NMOS pull-down devices, allowing the full INTV
to be applied to the gates during power MOSFET switch-
ing. Nonetheless, care should be taken to determine the
minimum gate drive supply voltage (INTV
choose the optimum power MOSFETs. Important param-
eters that can affect the minimum gate drive voltage are
the minimum input voltage (V
voltage, the Q
frequency.
operaTion
G
of the power MOSFETs, and the operating
IN
. A trade-off between the operat-
IN(MIN)
DS(ON)
), the LDO dropout
ratings. The gate
CC
) in order to
CC
voltage
If the input voltage V
to be in dropout, then the minimum gate drive supply
voltage is:
The LDO dropout voltage is a function of the total gate
drive current and the quiescent current of the IC (typically
3mA). A curve of dropout voltage vs output current for the
LDO is shown in Figure 2. The temperature coefficient of
the LDO dropout voltage is approximately 6000ppm/°C.
The total Q-current (I
of the controller quiescent current (3mA) and the total gate
charge drive current.
After the calculations have been completed, it is impor-
tant to measure the gate drive waveforms and the gate
driver supply voltage (INTV
conditions (low V
as from light load to full load) to ensure adequate power
MOSFET enhancement. Consult the power MOSFET data
sheet to determine the actual R
V
the component temperatures using an infrared camera
or thermal probe.
GS
V
I
Q(TOT)
, and verify your thermal calculations by measuring
INTVCC
Figure 2. INTV
1400
1200
1000
= I
600
800
400
200
= V
0
Q
0
IN(MIN)
+ Q
G(TOT)
IN
10
CC
, nominal V
IN
Q(TOT)
– V
LDO Dropout Voltage vs Current
INTV
is low enough for the INTV
• f
20
DROPOUT
CC
125°C
) flowing in the LDO is the sum
CC
LOAD (mA)
to PGND) over all operating
30
150°C
IN
DS(ON)
–40°C
and high V
LTC3862-2
25°C
85°C
40
for the measured
38622 F02
50
IN
, as well
15
CC
LDO
38622f