LTC3816_15 Datasheet, PDF (17/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
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.
Undervoltage 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_UVLO
=
1.2V
=
V_IN(UVLO)
R_ON1
R_ON1 + R_ON2
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-
SHUTDOWN
V_IN
R_{ON2 VRON}
1µA
+
R_{ON1 1.2V –}
LTC3816
ON
3816 F03
Figure 3. V_IN UVLO Circuit
3816f
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