Datasheet

LTC3816
18
3816f
applicaTions inForMaTion
down mode by pulling the VR
ON
pin below 0.65V. In the
shutdown mode, the internal circuitry and the INTV
CC
regulator are off and the supply current drops well below
100µA. When the VR
ON
pin voltage is between 0.65V and
1.2V, the INTV
CC
regulator and internal circuitry power up
but the driver outputs remain low.
TOPSIDE
MOSFET DRIVER SUPPLY
An external bootstrap capacitor, C
B
, connected from the
BOOST pin to the SW pin supplies the topside gate driver
as shown in Figure 1. Capacitor C
B
is charged though the
external diode, D
B
, from INTV
CC
when the SW pin is low.
When the topside MOSFET is turned on, the top driver
places the C
B
voltage across the gate source of the top
MOSFET. This enhances the MOSFET and turns on the
top switch. The switch node voltage, SW, rises to V
IN
and
the BOOST pin follows. With the topside MOSFET on, the
boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
The value of the boost capacitor, C
B
, needs to be at least
100 times that of the total input capacitance of the topside
MOSFET. The reverse breakdown of the external Schottky
diode, D
B
, must be greater than V
IN(MAX)
.
IMVP-6/IMVP-6.5 SELECTION AND V
BOOT
VOLTAGE
The LTC3816 can be configured to meet either IMVP-6
or IMVP-6.5 requirements. To select IMVP-6 operation,
short both I
MON
and PREI
MON
to INTV
CC
. At start-up, when
VR
ON
is asserted, the switcher output ramps to V
BOOT
=
1.2V regardless of the VID code. To configure IMVP-6.5
operation, connect a resistor from I
MON
to V
SS(SEN)
and
another resistor from PREI
MON
to I
TC
. The I
MON
and
PREI
MON
resistance set the I
MON
gain (see the I
MON
sec-
tion). The V
BOOT
voltage for IMVP-6.5 is 1.1V.
SOFT-START OPERATION
The start-up of V
OUT
is controlled by the LTC3816’s SS
pin. When the voltage at the SS pin is less than 1.3V, the
LTC3816 regulates the V
FB
voltage to the SS pin voltage
instead of 1.3V. This allows the user to program the soft-
start of the regulator output with a capacitor from the SS
pin to GND. An internal 1µA current source charges this
capacitor, creating a voltage ramp on the SS pin. As the
SS pin voltage rises from 0V to 1.3V, the output voltage,
V
OUT
, rises smoothly from 0V to its V
BOOT
value. Once
the soft-start interval is over, the internal current source
continues charging the SS capacitor until the SS poten-
tial is internally clamped at about 2.7V. For the IMVP-6
configuration, a 1000pF SS capacitor generates roughly
a 1.6ms start-up time. With the IMVP-6.5 configuration,
the start-up time is about 1.5ms.
During severe overload conditions, the LTC3816 discharges
the SS capacitor to lower the switcher output voltage. If the
potential at SS is forced below 0.3V, the controller reduces
its I
MAX
sourcing current from 10µA to 2.5µA and cuts the
short-circuit current to about 25% of its nominal value.
CURRENT
SENSE AND CURRENT LIMIT
The LTC3816 features an onboard cycle-by-cycle user-
programmable current limit circuit that controls the peak
inductor current. The I
MAX
pin has an internal 10µA pull-
up current source, allowing the maximum load current
I
LOAD(MAX)
to be programmed by a single external resistor
R
IMAX
connected between the I
MAX
and I
SENN
pins.
I
I R
R
I I I
L PEAK
IMAX IMAX
SENSE
LOAD MAX LIMIT L
( )
( ) (
•
=
< =
PPEAK
L IMAX IMAX
SENSE
L
I I R
R
I
)
–
•
–
∆ ∆
2 2
=
where I
L(PEAK)
is the peak inductor current, I
IMAX
is the I
MAX
pin pull-up current, R
SENSE
is the current sense resistor
value and ∆I
L
is the inductor ripple current.
∆I
f L
V
V
V
L
OSC
OUT
OUT
IN
=






1
1
•
–
Note that the output ripple current varies with the switch-
ing frequency, inductor value and duty cycle. Hence, the
current limit value should be checked on the application
board to ensure that I
LOAD(MAX)
< I
LIMIT
under all operating
conditions and temperature variations.
For current sensing using a low value sense resistor, the
sense resistor parasitic inductance must be considered to