LTC3859_15 Datasheet, PDF (28/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
on, the boost voltage is above the input supply: V_BOOST
= V_IN + V_INTVCC (V_BOOST = V_OUT + V_INTVCC for the boost
controller). The value of the boost capacitor C_B needs to
be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V_IN(MAX) for the buck
channels and V_OUT(MAX) for the boost channel.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
The topside MOSFET driver for the boost channel includes
an internal charge pump that delivers current to the boot-
strap capacitor from the BOOST3 pin. This charge current
maintains the bias voltage required to keep the top MOSFET
on continuously during dropout/overvoltage conditions.
The Schottky diode selected for the boost topside driver
should have a reverse leakage less than the available output
current the charge pump can supply under all operating
conditions. Curves displaying the available charge pump
current under different operating conditions can be found
in the Typical Performance Characteristics section.
Fault Conditions: Buck Current Limit and Current
Foldback
The LTC3859 includes current foldback for the buck
channels to help limit load current when the output is
shorted to ground. If the buck output falls below 70% of
its nominal output level, then the maximum sense volt-
age is progressively lowered from 100% to 40% of its
maximum selected value. Under short-circuit conditions
with very low duty cycles, the buck channel will begin
cycle skipping in order to limit the short-circuit current.
In this situation the bottom MOSFET will be dissipating
most of the power but less than in normal operation. The
short-circuit ripple current is determined by the minimum
on-time t_ON(MIN) of the LTC3859 (≈95ns), the input voltage
and inductor value:
DI_L(SC) = t_ON(MIN) (V_IN/L)
The resulting average short-circuit current is:
I_SC
=
40%
• I_LIM(MAX)
−
1
2
ΔI_L(SC)
28
Fault Conditions: Buck Overvoltage Protection
(Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the one of the buck
regulators rises much higher than nominal levels. The
crowbar causes huge currents to flow, that blow the fuse
to protect against a shorted top MOSFET if the short oc-
curs while the controller is operating.
A comparator monitors the buck output for overvoltage
conditions. The comparator detects faults greater than
10% above the nominal output voltage. When this condi-
tion is sensed, the top MOSFET of the buck controller is
turned off and the bottom MOSFET is turned on until the
overvoltage condition is cleared. The bottom MOSFET
remains on continuously for as long as the overvoltage
condition persists; if V_OUT returns to a safe level, normal
operation automatically resumes.
A shorted top MOSFET for the buck channel will result in
a high current condition which will open the system fuse.
The switching regulator will regulate properly with a leaky
top MOSFET by altering the duty cycle to accommodate
the leakage.
Fault Conditions: Over Temperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self heating on chip
(such as INTV_CC short to ground), the over temperature
shutdown circuitry will shut down the LTC3859. When the
junction temperature exceeds approximately 170°C, the
over temperature circuitry disables the INTV_CC LDO, caus-
ing the INTV_CC supply to collapse and effectively shutting
down the entire LTC3859 chip. Once the junction tempera-
ture drops back to approximately 155°C, the INTV_CC LDO
turns back on. Long term overstress (T_J > 125°C) should
be avoided as it can degrade the performance or shorten
the life of the part.
Phase-Locked Loop and Frequency Synchronization
The LTC3859 has an internal phase-locked loop (PLL)
comprised of a phase frequency detector, a lowpass filter,
and a voltage-controlled oscillator (VCO). This allows the
turn-on of the top MOSFET of controller 1 to be locked to
the rising edge of an external clock signal applied to the
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