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LTC3816_15 Datasheet, PDF (18/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs
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LTC3816
APPLICATIONS INFORMATION
down mode by pulling the VRON pin below 0.65V. In the
shutdown mode, the internal circuitry and the INTVCC
regulator are off and the supply current drops well below
100µA. When the VRON pin voltage is between 0.65V and
1.2V, the INTVCC regulator and internal circuitry power up
but the driver outputs remain low.
Topside MOSFET Driver Supply
An external bootstrap capacitor, CB, connected from the
BOOST pin to the SW pin supplies the topside gate driver
as shown in Figure 1. Capacitor CB is charged though the
external diode, DB, from INTVCC when the SW pin is low.
When the topside MOSFET is turned on, the top driver
places the CB 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 VIN and
the BOOST pin follows. With the topside MOSFET on, the
boost voltage is above the input supply:
VBOOST = VIN + VINTVCC
The value of the boost capacitor, CB, 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, DB, must be greater than VIN(MAX).
IMVP-6/IMVP-6.5 Selection and VBOOT Voltage
The LTC3816 can be configured to meet either IMVP‑6
or IMVP-6.5 requirements. To select IMVP-6 operation,
short both IMON and PREIMON to INTVCC. At start-up, when
VRON is asserted, the switcher output ramps to VBOOT =
1.2V regardless of the VID code. To configure IMVP‑6.5
operation, connect a resistor from IMON to VSS(SEN) and
another resistor from PREIMON to ITC. The IMON and
PREIMON resistance set the IMON gain (see the IMON sec-
tion). The VBOOT voltage for IMVP-6.5 is 1.1V.
Soft-Start Operation
The start-up of VOUT is controlled by the LTC3816’s SS
pin. When the voltage at the SS pin is less than 1.3V, the
LTC3816 regulates the VFB 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
18
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,
VOUT , rises smoothly from 0V to its VBOOT 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 IMAX 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 IMAX pin has an internal 10µA pull-
up current source, allowing the maximum load current
ILOAD(MAX) to be programmed by a single external resistor
RIMAX connected between the IMAX and ISENN pins.
IL(PEAK)
=
IIMAX •RIMAX
RSENSE
ILOAD(MAX)
< ILIMIT
= IL(PEAK)
–
∆IL
2
=
IIMAX •RIMAX
RSENSE
–
∆IL
2
where IL(PEAK) is the peak inductor current, IIMAX is the IMAX
pin pull-up current, RSENSE is the current sense resistor
value and ∆IL is the inductor ripple current.
∆IL
=
1
fOSC
•L
VOUT


1–
VOUT
VIN


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 ILOAD(MAX) < ILIMIT 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
3816f
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