ltc3862gn-1 Linear Technology Corporation, ltc3862gn-1 Datasheet - Page 14

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

LTC3862-1
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
frequency and the size of the power MOSFETs may need
to be made in order to maintain a reliable junction tem-
perature. Finally, it is important to verify the calculations
by performing a thermal analysis of the fi nal PCB using
an infrared camera or thermal probe. As an option, an
exernal 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 LTC3862-1’s internal thermal sen-
sor is intended to protect the device during momentary
overtemperature conditions. Continuous operation above
the specifi ed maximum operating junction temperature,
however, may result in device degradation.
Operation at Low Supply Voltage
The LTC3862-1 has a minimum input voltage of 8.5V, making
it a good choice for applications that require high voltage
power MOSFETs. The gate driver for the LTC3862-1 consists
of PMOS pull-up and NMOS pull-down devices, allowing
the full INTV
power MOSFET switching. Nonetheless, care should be
taken to determine the minimum gate drive supply voltage
(INTV
Important parameters that can affect the minimum gate
drive voltage are the minimum input voltage (V
the LDO dropout voltage, the Q
and the operating frequency.
OPERATION
14
CC
) in order to choose the optimum power MOSFETs.
CC
voltage to be applied to the gates during
IN
. A tradeoff between the operating
G
of the power MOSFETs,
IN(MIN)
),
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 coeffi cient 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
1200
1000
= I
800
600
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
) fl owing in the LDO is the sum
CC
LOAD (mA)
to PGND) over all operating
30
IN
DS(ON)
and high V
150°C
40
–40°C
for the measured
85°C
25°C
38621 F02
50
IN
, as well
CC
LDO
38621f