LTC3861-1 Datasheet, PDF (13/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Operation (Refer to Functional Diagram)
turn-on of the bottom MOSFET during a negative OC fault
condition. In this condition, the bottom MOSFET will be
turned on for 20ns every eight cycles to allow the driver IC
to recharge its topside gate drive capacitor. The negative
OC threshold is equal to –3/4 the positive OC threshold.
See the Applications Information section for guidelines
on setting these thresholds.
Excellent Transient Response
The LTC3861-1 error amplifiers are true operational ampli-
fiers, meaning that they have high bandwidth, high DC gain,
low offset and low output impedance. Their bandwidth,
when combined with high switching frequencies and low-
value inductors, allows the compensation network to be
optimized for very high control loop crossover frequencies
and excellent transient response. The 600mV internal ref-
erence allows regulated output voltages as low as 600mV
without external level-shifting amplifiers.
Line Feedforward Compensation
The LTC3861-1 achieves outstanding line transient re-
sponse using a feedforward correction scheme which
instantaneously adjusts the duty cycle to compensate for
changes in input voltage, significantly reducing output
overshoot and undershoot. It has the added advantage
of making the DC loop gain independent of input voltage.
Figure 2 shows how large transient steps at the input have
little effect on the output voltage.
V_IN
10V/DIV
I_L1
10A/DIV
I_L2
10A/DIV
V_OUT
50mV/DIV
AC-COUPLED
20µs/DIV
38611 F02
V_IN = 7V TO 14V IN 20µs I_LOAD = 20A
V_OUT = 1.2V
f_SW = 300kHz
Figure 2.
Remote Sense Differential Amplifier
The LTC3861-1 includes a low offset, unity gain, high
bandwidth differential amplifier for differential output
sensing. Output voltage accuracy is significantly improved
by removing board interconnection losses from the total
error budget.
The LTC3861-1 differential amplifier has a typical output
slew rate of 45V/µs, bandwidth of 40MHz, input referred
offset < 2mV and a typical maximum output voltage of V_CC
– 1V. The amplifier is configured for unity gain, meaning
that the differential voltage between V_SNSP and V_SNSN is
translated to V_SNSOUT, relative to SGND.
Shutdown Control Using the RUN Pins
The two channels of the LTC3861-1 can be independently
enabled using the RUN1 and RUN2 pins. When both pins
are driven low, all internal circuitry, including the internal
reference and oscillator, are completely shut down. When
the RUN pin is low, the respective COMP pin is actively
pulled down to ground. In a multiphase operation when
the COMP pins are tied together, the COMP pin is held
low until all the RUN pins are enabled. This ensures a
synchronized start-up of all the channels. A 1.5μA pull-up
current is provided for each RUN pin internally. The RUN
pins remain high impedance up to V_CC.
Undervoltage Lockout
To prevent operation of the power supply below safe in-
put voltage levels, both channels are disabled when V_CC
is below the undervoltage lockout (UVLO) threshold
(2.9V falling, 3V rising). If a RUN pin is driven high, the
LTC3861-1 will start up the reference to detect when
V_CC rises above the UVLO threshold, and enable the
appropriate channel.
Overvoltage Protection
If the output voltage rises to more than 10% above the
set regulation value, which is reflected as a V_FB voltage
of 0.66V or above, the LTC3861-1 will force the PWM
output low to turn on the bottom MOSFET and discharge
the output. Normal operation resumes once the output
is back within the regulation window. However, if the re-
verse current flowing from V_OUT back through the bottom
38611f
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