LTC3546IUFD#PBF Linear Technology, LTC3546IUFD#PBF Datasheet - Page 18

• Manufacturer Part Number: LTC3546IUFD#PBF
• Description: IC BUCK SYNC ADJ 1A/2A DL 28QFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3546IUFD#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 5 V
• Current - Output: 1A, 2A
• Frequency - Switching: 2.25MHz, 0.75MHz ~ 4MHz
• Voltage - Input: 2.25 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-QFN
• Dc To Dc Converter Type: Step Down
• Pin Count: 28
• Input Voltage: 5.5V
• Output Voltage: 0.6 to 5V
• Switching Freq: 750 TO 4000/2900KHz
• Output Current: 2A
• Efficiency: 96%
• Package Type: QFN EP
• Output Type: Adjustable
• Switching Regulator: Yes
• Load Regulation: 0.2%
• Line Regulation: 0.2%/V
• Mounting: Surface Mount
• Input Voltage (min): 2.25V
• Operating Temperature Classification: Automotive
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
LTC3546
Keeping the current small (<20μA) in these resistors
maximizes effi ciency, but making the current too small
may allow stray capacitance to cause noise problems and
reduce the phase margin of the error amp loop.
To improve the frequency response, a feedforward capacitor
C
100pf. Great care should be taken to route the V
from noise sources, such as the inductor or an SW line.
Shutdown, Soft-Start and Tracking Start-Up
The LTC3546 start-up works by comparing two inputs, an
internal 1.2ms linear soft-start ramp and the TRACK/SS
pin. Whichever input is lower in voltage is the controlling
voltage used for start-up. The internal start-up ramps
to 0.6V in 1.2ms. If a slower start-up is desired, the
TRACK/SS pin has a 1.15μA pull up current so a start-up
ramp rate can be programmed with an external capacitor,
or a voltage divider from another signal can be applied
to the TRACK/SS pin.
During start-up the controlling voltage must rise above
120mV before the output will start switching.
When the RUN pin is low, both the internal 1.2ms soft-start
ramp and the TRACK/SS pin are pulled to ground. When
the RUN pin is pulled high, both the internal soft-start ramp
and the TRACK/SS pin are released. From the time when
the RUN pin is asserted until the controlling voltage reaches
0.6V, the regulator is in the start-up state. In this state, the
error amplifi er will compare the feedback signal at V
the controlling voltage (the lower of either the TRACK/SS
voltage or the internal ramp voltage) and the regulator
will force them to be equal. In this state, the mode of the
regulator is forced to pulse skipping. The regulator will
continue in this manner until the voltage on the control-
ling voltage rises above 0.6V. Once the controlling ramp
signal is above 0.6V the error amplifi er uses the internal
0.6V reference and the operational mode will switch to
the mode set by the SYNC/MODE pin.
If the TRACK/SS pin is ramped down after start-up, the
error amplifi er will compare the feedback signal at V
to the voltage on the TRACK/SS pin once the TRACK/SS
voltage drops 6% below the internal reference voltage of
0.6V (0.564V). The regulator will try to force the V
18
FF
may also be used. Typical values used here are 10pf to
FB
node away
FB
voltage
FB
to
FB
to equal the TRACK/SS voltage if there is suffi cient load
current to pull the output low at this rate, otherwise the
output will ramp down at the discharge rate of the output
capacitor. Once the TRACK/SS voltage drops below about
100mV all switching functions cease and the regulator is
forced back into pulse-skipping mode. The operational
mode while TRACK/SS is ramping down is set by the
MODE/SYNC pin.
To use the internal 1.2ms linear soft-start controlling
voltage leave the TRACK/SS pin fl oating. By fl oating the
TRACK/SS pin the internal 1.15μA pull up current will pull
the TRACK/SS pin up faster than the internal 1.2ms ramp.
Care must be taken to insure the TRACK/SS ramp up time
(from 0V to 0.6V) is much shorter than the internal 1.2ms
ramp time. Parasitic capacitance on this pin should be
much smaller than:
or
An externally controlled soft-start ramp is obtained when
an external capacitor is connected from the TRACK/SS
pin to ground and its ramp rate is slower than the internal
soft-start ramp. In this confi guration, soft-start times longer
than 1.2ms can be achieved. When RUN is pulled high, the
internal 1.15μA current source charges the external capaci-
tor linearly from 0V. While the TRACK/SS pin is below 0.6V
the error amplifi er forces the regulator to drive the V
the voltage on the TRACK/SS pin. Once the V
0.6V the regulator switches to the internal 0.6V reference.
The ramp-up time for the output is calculated as:
For this equation to be valid, the ramp time must be greater
than 1.2ms thus:
or
C
C
C
C
t
RAMP
PARASITICTRACK/SS
TRACK/SS
PARASITICTRACK SS
TRACK SS
=
/
C
≥ 2.3nF
TRACK SS
≥
1 15
1 15
.
.
/
/
µA
µA
0 6
<< 2.3nF
•
.
< <
•
0 6
V
1 2
.
.
1 15
V
.
ms
µA
0 6
.
•
V
1 2
.
ms
FB
pin reaches
FB
pin to
3546fb