LTC3890
applicaTions inForMaTion
Fault Conditions: Current Limit and Current Foldback
Phase-Locked Loop and Frequency Synchronization
The LTC3890 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
PLLIN/MODEpin.Theturn-onofcontroller2’stopMOSFET
is thus 180 degrees out of phase with the external clock.
The phase detector is an edge sensitive digital type that
provides zero degrees phase shift between the external
and internal oscillators. This type of phase detector does
not exhibit false lock to harmonics of the external clock.
The LTC3890 includes current foldback to help limit
load current when the output is shorted to ground. If
the output voltage falls below 70% of its nominal output
level, then the maximum sense voltage is progressively
lowered from 100% to 45% of its maximum selected
value. Under short-circuit conditions with very low duty
cycles, the LTC3890 will begin cycle skipping in order to
limittheshort-circuitcurrent. Inthissituationthebottom
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
, of the
ON(MIN)
LTC3890 (≈95ns), the input voltage and inductor value:
If the external clock frequency is greater than the internal
oscillator’sfrequency,f
,thencurrentissourcedcontinu-
OSC
V
L
IN
∆IL(SC) = t
ON(MIN)
ously from the phase detector output, pulling up the VCO
input. When the external clock frequency is less than f
,
OSC
current is sunk continuously, pulling down the VCO input.
If the external and internal frequencies are the same but
exhibit a phase difference, the current sources turn on for
an amount of time corresponding to the phase difference.
The voltage at the VCO input is adjusted until the phase
and frequency of the internal and external oscillators are
identical. At the stable operating point, the phase detector
output is high impedance and the internal filter capacitor,
The resulting average short-circuit current is:
1
2
ISC = 45% •ILIM(MAX) – ∆IL(SC)
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
muchhigherthannominallevels.Thecrowbarcauseshuge
currents to flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
C , holds the voltage at the VCO input.
LP
1000
900
800
700
600
500
400
300
200
100
A comparator monitors the output for overvoltage condi-
tions. The comparator detects faults greater than 10%
above the nominal output voltage. When this condition
is sensed, the top MOSFET is turned off and the bottom
MOSFET is turned on until the overvoltage condition is
cleared. ThebottomMOSFETremainsoncontinuouslyfor
aslongastheovervoltageconditionpersists;ifV returns
to a safe level, normal operation automatically resumes.
OUT
0
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
AshortedtopMOSFETwillresultinahighcurrentcondition
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.
3890 F10
Figure 10. Relationship Between Oscillator Frequency
and Resistor Value at the FREQ Pin
3890fb
23
Linear Systems [ Linear Systems ]
