LM2743MTC National Semiconductor, LM2743MTC Datasheet - Page 10

• Manufacturer Part Number: LM2743MTC
• Description: SYNC SW CONTROL, 3V:6V, POWERWISE
• Manufacturer: National Semiconductor

Specifications of LM2743MTC

• Primary Input Voltage: 16V
• No. Of Outputs: 1
• Output Voltage: 13.5V
• Output Current: 20A
• Voltage Regulator Case Style: TSSOP
• No. Of Pins: 14
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Application Information
NORMAL OPERATION
While in normal operation mode, the LM2743 regulates the
output voltage by controlling the duty cycle of the high side
and low side FETs.
The equation governing output voltage is:
The PWM frequency is adjustable between 50kHz and
2MHz and is set by an external resistor, R
FREQ pin and ground. The resistance needed for a desired
frequency is approximately:
where a
the coefficients, F
the resistance in kΩ.
SS/TRACK
When the LM2743 is used for sequencing purposes, some
care has to be taken. Once the shutdown voltage goes
above V
in Figure 2. This current is used to create an internal hyster-
esis (170mV); however, high external impedances will affect
the SD pin level as well. The external impedance must be
lower than 10kΩ to work properly without glitching the qui-
escent current of the chip. In that scenario the SS current will
turn on and off during the glitching or most likely no switching
will occur at all, due to the SS voltage being very low.
2
STBY-IH
= 0.206375, a
FIGURE 1. Start-Up Behavior
, a 17µA pull-up current is activated as shown
OSC
is the frequency in Hz, and R
1
= 3.691525, a
0
FADJ
= (-0.076875) are
(Continued)
, between the
20095204
FADJ
is
10
The LM2743 is also adequate for tracking purposes through
the SS/TRACK pin. The tracking circuit, in the design ex-
amples below, contains a Master Power Supply with V
5V and an LM2743 with V
Three cases are described:
1. Both output voltages, V
2. Both output voltages rise together at the same rate until
3. Output voltage V
The calculation of the feedback resistors Figure 3 for all
cases is based on the Tracking Equation. Since V
V
Case 1: Rise together
Both, V
values at the same time as shown in Figure 4. This means
that V
OUT2
reaching their final values at the same time,
V
starts.
OUT2
OUT2
=1.8V, and R
OUT1
FIGURE 3. Block Diagram of Case 1
= 1.8V, and finally
rises at slower rate then V
and V
V
FIGURE 2. SD Pin Logic
SS
OUT2
FB2
= V
OUT2
=10kΩ, then the R
, start rising, and reach their nominal
OUT1
OUT2
starts rising 3.24ms after V
OUT1
*(R
=1.8V.
3
and V
/(R
3
OUT1
+R
OUT2
4
FB1
))
.
, rise together,
becomes 5kΩ.
20095207
FB
20095206
OUT1
=0.6V,
OUT1
=