LM27313XMF/NOPB National Semiconductor, LM27313XMF/NOPB Datasheet - Page 6

• Manufacturer Part Number: LM27313XMF/NOPB
• Description: IC CONV BOOST SW 1.6MHZ SOT23-5
• Manufacturer: National Semiconductor
• Type: Step-Up (Boost)r

Specifications of LM27313XMF/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5 ~ 28 V
• Current - Output: 800mA
• Frequency - Switching: 1.6MHz
• Voltage - Input: 2.7 ~ 14 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Power - Output: 2.5W
• Package: 5SOT-23
• Minimum Input Voltage: 2.7 V
• Maximum Input Voltage: 14 V
• Switching Frequency: 1600(Typ) KHz
• Operating Supply Voltage: 2.7 to 14 V
• Output Type: Adjustable
• Output Voltage: 4 to 28 V
• Efficiency: 80 %
• Dc To Dc Converter Type: Step Up
• Pin Count: 5
• Input Voltage: 14V
• Switching Freq: 1600KHz
• Package Type: SOT-23
• Switching Regulator: Yes
• Mounting: Surface Mount
• Input Voltage (min): 2.7V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• Primary Input Voltage: 5V
• No. Of Outputs: 1
• Output Current: 1.25A
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Filter Terminals: SMD
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM27313XMFTR

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Block Diagram
Theory of Operation
The LM27313 is a switching converter IC that operates at a
fixed frequency of 1.6 MHz using current-mode control for fast
transient response over a wide input voltage range and in-
corporate pulse-by-pulse current limiting protection. Because
this is current mode control, a 50 mΩ sense resistor in series
with the switch FET is used to provide a voltage (which is
proportional to the FET current) to both the input of the pulse
width modulation (PWM) comparator and the current limit
amplifier.
At the beginning of each cycle, the S-R latch turns on the FET.
As the current through the FET increases, a voltage (propor-
tional to this current) is summed with the ramp coming from
the ramp generator and then fed into the input of the PWM
comparator. When this voltage exceeds the voltage on the
other input (coming from the Gm amplifier), the latch resets
and turns the FET off. Since the signal coming from the Gm
amplifier is derived from the feedback (which samples the
voltage at the output), the action of the PWM comparator
constantly sets the correct peak current through the FET to
keep the output voltage in regulation.
Q1 and Q2 along with R3 - R6 form a bandgap voltage refer-
ence used by the IC to hold the output in regulation. The
currents flowing through Q1 and Q2 will be equal, and the
feedback loop will adjust the regulated output to maintain this.
Because of this, the regulated output is always maintained at
a voltage level equal to the voltage at the FB node "multiplied
up" by the ratio of the output resistive divider.
The current limit comparator feeds directly into the flip-flop,
that drives the switch FET. If the FET current reaches the limit
threshold, the FET is turned off and the cycle terminated until
the next clock pulse. The current limit input terminates the
pulse regardless of the status of the output of the PWM com-
parator.
6
Application Information
SELECTING THE EXTERNAL CAPACITORS
The LM27313 requires ceramic capacitors at the input and
output to accommodate the peak switching currents the part
needs to operate. Electrolytic capacitors have resonant fre-
quencies which are below the switching frequency of the
device, and therefore can not provide the currents needed to
operate. Electrolytics may be used in parallel with the ceram-
ics for bulk charge storage which will improve transient re-
sponse.
When selecting a ceramic capacitor, only X5R and X7R di-
electric types should be used. Other types such as Z5U and
Y5F have such severe loss of capacitance due to effects of
temperature variation and applied voltage, they may provide
as little as 20% of rated capacitance in many typical applica-
tions. Always consult capacitor manufacturer’s data curves
before selecting a capacitor. High-quality ceramic capacitors
can be obtained from Taiyo-Yuden, AVX, and Murata.
SELECTING THE OUTPUT CAPACITOR
A single ceramic capacitor of value 4.7 µF to 10 µF will provide
sufficient output capacitance for most applications. For output
voltages below 10V, a 10 µF capacitance is required. If larger
amounts of capacitance are desired for improved line support
and transient response, tantalum capacitors can be used in
parallel with the ceramics. Aluminum electrolytics with ultra
low ESR such as Sanyo Oscon can be used, but are usually
prohibitively expensive. Typical AI electrolytic capacitors are
not suitable for switching frequencies above 500 kHz due to
significant ringing and temperature rise due to self-heating
from ripple current. An output capacitor with excessive ESR
can also reduce phase margin and cause instability.
SELECTING THE INPUT CAPACITOR
An input capacitor is required to serve as an energy reservoir
for the current which must flow into the inductor each time the
switch turns ON. This capacitor must have extremely low ESR
and ESL, so ceramic must be used. We recommend a nom-
20216803