ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 23

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to fl ow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
A comparator monitors the output for overvoltage con-
ditions. The comparator (OV) detects overvoltage faults
greater than 7.5% 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. The output of this comparator is
only latched by the overvoltage condition itself and will
therefore allow a switching regulator system having a poor
PC layout to function while the design is being debugged.
The bottom MOSFET remains on continuously for as long
as the OV condition persists; if V
normal operation automatically resumes. A shorted top
MOSFET 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.
Phase-Locked Loop and Frequency Synchronization
The LTC3728 has a phase-locked loop comprised of an
internal voltage controlled oscillator and phase detector.
This allows the top MOSFET turn-on to be locked to the
rising edge of an external source. The frequency range
of the voltage controlled oscillator is ± 50% around the
center frequency f
pin of 1.2V corresponds to a frequency of approximately
400kHz. The nominal operating frequency range of the
LTC3728 is 250kHz to 550kHz.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the ex-
O
. A voltage applied to the PLLFLTR
OUT
returns to a safe level,
ternal and internal oscillators. This type of phase detector
will not lock up on input frequencies close to the harmonics
of the VCO center frequency. The PLL hold-in range, Δf
is equal to the capture range, Δf
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
fi lter network on the PLLFLTR pin.
If the external frequency (f
frequency f
up the PLLFLTR pin. When the external frequency is less
than f
PLLFLTR pin. 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. Thus the voltage on the PLLFLTR pin is adjusted
until the phase and frequency of the external and internal
oscillators are identical. At this stable operating point the
phase comparator output is open and the fi lter capacitor
C
driven from a low impedance source such as a logic gate
located close to the pin. When using multiple LTC3728’s
(or LTC3729’s, as shown in Figure 14) for a phase-locked
system, the PLLFLTR pin of the master oscillator should be
biased at a voltage that will guarantee the slave oscillator(s)
ability to lock onto the master’s frequency. A DC voltage
of 0.7V to 1.7V applied to the master oscillator’s PLLFLTR
pin is recommended in order to meet this requirement.
The resultant operating frequency can range from 300kHz
to 470kHz.
The loop fi lter components (C
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The fi lter
components C
acquires lock. Typically R
to 0.1μF .
LP
Δf
holds the voltage. The LTC3728 PLLIN pin must be
H
OSC
= Δf
, current is sunk continuously, pulling down the
C
OSC
= ±0.5 f
, current is sourced continuously, pulling
LP
and R
O
(250kHz-550kHz)
LP
PLLIN
LP
determine how fast the loop
) is greater than the oscillator
=10kΩ and C
LP
C:
, R
LP
) smooth out the
LTC3728
LP
is 0.01μF
23
3728fd
H
,