Datasheet

LTC3862
14
3862fc
For more information www.linear.com/LTC3862
operaTion
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
IN
. A tradeoff between the operating
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 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’s internal thermal sensor is
intended to protect the device during momentary over
-
temperature conditions. Continuous operation above
the specified maximum operating junction temperature,
however, may result in device degradation.
Operation at Low Supply Voltage
The L
TC3862 has a minimum input voltage of 4V, making it
a good choice for applications that experience low supply
conditions. The gate driver for the LTC3862 consists of
PMOS pull-up and NMOS pull-down devices, allowing
the full INTV
CC
voltage to be applied to the gates during
power MOSFET switching. Nonetheless, care should be
taken to determine the minimum gate drive supply voltage
(INTV
CC
) in order to choose the optimum power MOSFETs.
Important parameters that can affect the minimum gate
drive voltage are the minimum input voltage (V
IN(MIN)
),
the LDO dropout voltage, the Q
G
of the power MOSFETs,
and the operating frequency.
If the input voltage V
IN
is low enough for the INTV
CC
LDO
to be in dropout, then the minimum gate drive supply
voltage is:
V
INTVCC
= V
IN(MIN)
– V
DROPOUT
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
Q(TOT)
) flowing in the LDO is the sum
of the controller quiescent current (3mA) and the total gate
charge drive current.
I
Q(TOT)
= I
Q
+ Q
G(TOT)
• f
After the calculations have been completed, it is important
to measure the gate drive waveforms and the gate driv
-
er supply voltage (INTV
CC
to PGND) over all operating
conditions (low V
IN
, nominal V
IN
and high V
IN
, as well
as from light load to full load) to ensure adequate power
MOSFET enhancement. Consult the power MOSFET data
sheet to determine the actual R
DS(ON)
for the measured
V
GS
, and verify your thermal calculations by measuring
the component temperatures using an infrared camera
or thermal probe.
INTV
CC
LOAD (mA)
0
DROPOUT VOLTAGE (mV)
600
800
1000
30
50
3862 F02
400
200
0
10 20 40
1200
1400
1600
85°C
25°C
–40°C
125°C
150°C
Figure 2. INTV
CC
LDO Dropout Voltage vs Current