LTC3728LEGN-1#PBF Linear Technology, LTC3728LEGN-1#PBF Datasheet - Page 20

• Manufacturer Part Number: LTC3728LEGN-1#PBF
• Description: IC REG SYNC DUAL 28-SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LEGN-1#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3728L-1
ent temperature, conditions that cause the highest power
dissipation in the top MOSFET.
Each controller includes current foldback to help further
limit load current when the output is shorted to ground.
The foldback circuit is active even when the overload
shutdown latch described above is overridden. If the
output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
75mV to 17mV. Under short-circuit conditions with very
low duty cycles, the controller 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
of each controller (typically 100ns), the input voltage and
inductor value:
The resulting short-circuit current is:
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 controller
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
20
ΔI
I
SC
L(SC)
=
R
25mV
= t
SENSE
ON(MIN)
–
2
1
(V
I
IN
L(SC)
/L)
ON(MIN)
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 IC has a phase-locked loop comprised of an internal
voltage controlled oscillator and phase detector. This al-
lows 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 corresponds to a frequency of approximately 400kHz.
The nominal operating frequency range of the IC is 260kHz
to 550kHz.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the ex-
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
cillator frequency f
Δf
H
= Δf
C
O
= ±0.5 f
. A voltage of 1.2V applied to the PLLFLTR
0SC
O
(260kHz-550kHz)
, current is sourced continuously,
PLLIN
OUT
C:
) is greater than the os-
returns to a safe level,
3728l1fc
H
,