LTC3860_15 Datasheet, PDF (11/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
OPERATION (Refer to Functional Diagram)
Excellent Transient Response
The LTC3860 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 LTC3860 achieves outstanding line transient response
using a feedforward correction scheme which instanta-
neously 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_OUT
50mV/DIV
I_L
2A/DIV
V_IN
5V/DIV
COMP1
100mV/DIV
20μs/DIV
V_OUT = 1.2V
V_IN STEP = 7V TO 14V
Figure 2
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Remote Sense Differential Amplifier
The LTC3860 includes a low offset, unity gain, high
bandwidth differential amplifier for remote output sens-
ing. Output voltage accuracy is significantly improved
by removing board interconnection losses from the total
error budget.
The LTC3860 differential amplifier has a typical output
slew rate of 45V/μs, bandwidth of 20MHz, 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 LTC3860 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. A 1.5μA
pull-up current is provided for each RUN pin internally.
The RUN pins remain low impedance up to V_CC. From V_CC
to 6V, they may sink some current.
Undervoltage Lockout
To prevent operation of the power supply below safe input
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 LTC3860 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 LTC3860 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 reverse
current flowing from V_OUT back through the bottom power
MOSFET to PGND is greater than 3/4 the positive OC
threshold, the NOC comparator trips and shuts off the
bottom power MOSFET to protect it from being destroyed.
This scenario can happen when the LTC3860 tries to start
into a precharged load, higher than the OV threshold. As
a result, the bottom switch turns on until the amount of
reverse current trips the NOC comparator threshold.
Internal Soft-Start
By default, the start-up of each channel’s output voltage
is normally controlled by an internal soft-start ramp. The
internal soft-start ramp represents a noninverting input
to the error amplifier. The FB pin is regulated to the lower
of the error amplifier’s three noninverting inputs (the
internal soft-start ramp for that channel, the TRACK/SS
pin or the internal 600mV reference). As the ramp volt-
age rises from 0V to 0.6V over approximately 2ms, the
output voltage rises smoothly from its prebiased value to
its final set value.
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