Datasheet

LTC3816
17
3816f
applicaTions inForMaTion
LDO, INTV
CC
/EXTV
CC
POWER SUPPLY
The LTC3816 is designed to operate with a wide range of
V
IN
input voltages. The IC includes a 5.2V LDO to power
the driver and control circuits. The LDO output, INTV
CC
should be bypassed with a minimum 4.7µF low ESR
ceramic capacitor. The INTV
CC
regulator can supply up to
50mA of total LTC3816 quiescent current, I
Q(TOT)
, which
consists of the static supply current, I
Q
, and the current
required to charge the gate capacitance, Q
G(TOT)
, of the
top and bottom power MOSFETs.
I
Q(TOT)
= I
Q
+ Q
G(TOT)
• f
OSC
P
DISS
= V
IN
• (I
Q
+ Q
G(TOT)
• f
OSC
)
T
J
= T
A
+ P
DISS
• θ
JA
The value of Q
G(TOT)
can be obtained from the MOSFET
data sheets. For high V
IN
and high frequency operation,
care must be taken to ensure that the maximum junction
temperature T
JMAX
of the IC is never exceeded.
When the EXTV
CC
pin is left open or tied to a voltage less
than 4.5V, the 5.2V LDO powers INTV
CC
. If EXTV
CC
is taken
above 4.5V, the LDO is turned off and an internal switch
connects INTV
CC
to EXTV
CC
. Do not apply greater than 6V
to the EXTV
CC
pin, and ensure that EXTV
CC
< V
IN
+ 0.3V
unless EXTV
CC
is shorted to the V
IN
supply. Using the
EXTV
CC
pin allows INTV
CC
to be powered from an external
source reducing LDO losses and improving the regulator
efficiency, especially at high V
IN
. When the EXTV
CC
pin is
used, the chip power dissipation reduces to:
P
DISS
= V
EXTVCC
• (I
Q
+ Q
G(TOT)
• f
OSC
)
If the V
IN
supply is low enough for the INTV
CC
LDO to enter
dropout, the output voltage of the LDO becomes:
V
INTVCC(DROPOUT)
= V
IN
– V
DROPOUT
The LDO dropout voltage is a function of the total quies-
cent current I
Q(TOT)
, V
IN
voltage and junction temperature.
The temperature coefficient of the LDO dropout voltage
is approximately 6400ppm/°C. To enable proper opera-
tion, make sure that the LDO output voltage meets the
INTV
CC
undervoltage and minimum MOSFET gate driver
requirements. If V
IN
is connected to a fixed 5V supply, it is
advisable to short EXTV
CC
to V
IN
. In this case, the INTV
CC
output voltage becomes:
V
INTVCC(EXTVCC)
= V
EXTVCC
– I
Q(TOT)
• R
EXTVCC
where R
EXTVCC
is the internal EXTV
CC
switch on-resistance.
It has a typical value of 2Ω at 25°C and has a temperature
coefficient of approximately 4000ppm/°C.
U
NDER
VOLTAGE LOCKOUT AND SHUTDOWN
A precision undervoltage lockout (UVLO) comparator
monitors the INTV
CC
voltage and enables soft-start opera-
tion once INTV
CC
is above 3.9V. For power supplies that
start-up slowly, the gate drivers could begin switching
when V
IN
is well below its steady-state value. The high
inrush current through the input power cable could cause
the V
IN
supply to dip below the UVLO threshold and result
in hiccup operation at start-up. This problem can be eas-
ily overcome by adding a V
IN
UVLO function as shown in
Figure 3. Connect an external resistive divider from V
IN
to
VR
ON
. Set the resistive divider according to the following
equation:
V V V
R
R R
UVLO IN UVLO
ON
ON ON
= =
+
1 2
1
1 2
.
( )
where V
IN(UVLO)
is the desired V
IN
UVLO threshold. The
resistances are normally chosen so that the error caused
by the internal 1µA pull-up current has a negligible effect
on the UVLO threshold. Be careful not to allow the resistive
divider output voltage to exceed the 6V maximum rating
of the VR
ON
pin.
If the external resistive divider is not used, upon power-
up, the VR
ON
pin is pulled up by an internal 1µA pull-up
current. The LTC3816 can be put into a low power shut-
Figure 3. V
IN
UVLO Circuit
+
–
SHUTDOWN
R
ON2
V
IN
VR
ON
1µA
LTC3816
ON
R
ON1
1.2V
3816 F03