LTC3811_15 Datasheet, PDF (17/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
OPERATION (Refer to the Functional Diagram)
If the internal LDO is supplying power to the gate driver
and the input voltage is low enough for the LDO to be in
dropout, then the minimum gate drive supply voltage is:
V_DRVCC = 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
10mA). A curve of dropout voltage vs output current for the
LDO is shown in the Typical Performance Characteristics.
The temperature coefficient of the LDO dropout voltage is
approximately +4000ppm/ºC.
The total Q-current (I_Q(TOT)) flowing in the LDO is the sum
of the controller quiescent current (typically 10mA) and
the total gate charge drive current.
I_Q(TOT) = I_Q + Q_G(TOT) • f
If an external supply is being used to supply power to the
gate driver through the EXTV_CC pin, then the minimum
gate drive supply voltage is:
V_DRVCC = V_EXTVCC – I_Q(TOT) • R_EXTVCC
The resistance of the internal EXTV_CC PMOS switch is
typically 5Ω at 25°C and has a temperature coefficient of
approximately 3400ppm/°C.
After the calculations have been completed, it is important
to measure the gate drive waveforms (BG-to-PGND and
TG-to-SW) and the gate driver supply voltage (DRV_CC-to-
PGND) over all operating conditions (low V_IN, mid 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.
On/Off Control Using the RUN Pin
The two channels of the LTC3811 can be independently
turned on and off using the RUN1 and RUN2 pins. Pull-
ing either of these pins low shuts down the main control
loop for that channel. Pulling both pins low disables both
controllers and most of the internal circuitry, including the
DRV_CC low dropout regulator (LDO). In shutdown mode
(both RUN pins low) the LTC3811 typically draws only
20μA of current.
The RUN pins may be externally pulled up or driven di-
rectly by logic. Be careful not to exceed the 7V absolute
maximum rating on this pin.
Soft-Start and Tracking Using the SS/TRACK Pin
The start-up of each controller’s output voltage V_OUT
is normally controlled by the voltage on the SS/TRACK
pin for that channel. The SS/TRACK pin represents a
2nd noninverting input to the error amplifier, as shown
in Figure 3. The error amplifier is configured so that the
lower of the two noninverting inputs (the SS/TRACK pin
or the 0.6V reference) controls the feedback loop. That
is, when the voltage on the SS/TRACK pin is less than the
0.6V internal reference, the LTC3811 regulates the FB pin
voltage to be approximately equal to the SS/TRACK pin
voltage instead of the internal 0.6V reference. This allows
the user to connect a capacitor from the SS/TRACK pin to
SGND to program the soft-start of the power supply output.
An internal 2.5μA current source charges this capacitor,
creating a voltage ramp on the SS/TRACK pin. As the SS/
TRACK pin voltage rises from 0V to 0.6V, the output volt-
age, V_OUT, rises smoothly from 0V to its final value. Once
the soft-start interval is over, the internal 2.5μA current
source will continue to charge the SS/TRACK capacitor
up to a maximum voltage equal to INTV_CC.
Alternately, the SS/TRACK pin can be used to force the
start-up of V_OUT to track the voltage of another supply.
Typically, this requires connecting the SS/TRACK pin to
an external divider from the other supply to ground (see
Applications Information).
INTV_CC
INTV_CC
Q2 Q4
V_FB
Q3
INTV_CC
2.5μA
SS/TRACK
0.6V Q1 Q5
SHDN
M1
3811 F03
Figure 3. Simplified LTC3811 Error Amplifier Input Stage
3811f
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