LTC3546IUFD#PBF Linear Technology, LTC3546IUFD#PBF Datasheet - Page 19

• 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
The LTC3546 can also track an external voltage during
startup by using an external voltage divider to the TRACK/SS
pin, and also insuring that the ramp rate on the TRACK/SS
pin is slower than the internal 1.2ms ramp rate.
As indicated in Figure 3, a resistor divider from an external
voltage can be connected to the TRACK/SS pin to allow
the start-up of V
voltage V
For V
Coincident tracking is where V
implement coincident tracking R1X in Figure 3 is set to the
same value as R1 and R2X to the value of R2. Coincident
tracking is illustrated in Figure 4. The voltage at TRACK/SS
when V
margin above the 0.6V reference voltage).
Ratiometric tracking is where V
but rather, it is set to some fractional value of V
implement ratiometric tracking (as illustrated in Figure
5), set R1X in Figure 3 to the same value as R1 and R2X
to the value of R2 + ΔR. The ΔR added to R2 should be
suffi cient so that the TRACK/SS voltage is ≥ 0.8V when
V
The internal 1.15μA pull up current on TRACK/SS can cause
a tracking error at V
TRACK/SS. For example, if a 59k resistor is chosen for
R2X, the R2X current will be about 10μA (0.6V/59k). In this
case, the 1.15μA internal current source will cause about
11% (1.15μA/10μA) tracking error, which is about 66mV
referred to V
If a better tracking accuracy is required, the value of R2X
can be reduced or the 1.15μA current can be taken into
account in the equations.
X
V
is at its fi nal value.
OUT
TRACKSS
X
=
X
is at its fi nal value should be ≥0.8V (suffi cient
.
V
X
Figure 3. Tracking External Voltages
FB
< 0.6V
•
. This is acceptable for most applications.
R X R X
OUT
1
V
R X
X
R1X
R2X
+
OUT
2
to ratiometrically track an external
2
V
TRACK/SS
when using a resistor divider on
LTC3546
•
R
1
OUT
3546 F03
R
V
+
FB
OUT
2
R
= V
2
R1
R2
V
≠ V
OUT
X
during startup. To
X
during startup
X
. To
Table 2 summarizes the different states in which the
TRACK/SS can be used.
Table 2. The States of the TRACK/SS Pin
TRACK/SS PIN
Capacitor to Ground
Floating
Resistor Divider
Regardless of the mode implemented, the TRACK/SS pin
should never be pulled high externally as this will result
in excessive current during shutdown.
The LTC3546 can smoothly handle starting up into a pre-
biased output. The tracking function will pick up from the
pre-biased voltage and ramp the output up from there.
Mode Selection
The SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connecting
this pin to V
IN
enables Burst Mode operation for both regula-
Figure 5. Ratiometric Tracking
Figure 4. Coincident Tracking
RESULT
External Soft-Start
Internal Soft-Start
V
OUT
TIME
TIME
Tracking an External Voltage V
3546 F04
3546 F05
LTC3546
V
V
V
V
X
OUT
X
OUT
19
X
3546fb