LM27313XMF/NOPB National Semiconductor, LM27313XMF/NOPB Datasheet - Page 9

• 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

DESIGN PARAMETERS V
The value of the FET "ON" voltage (referred to as V
equations) is dependent on load current. A good approxima-
tion can be obtained by multiplying the "ON Resistance" of
the FET times the average inductor current.
FET on resistance increases at V
the internal N-FET has less gate voltage in this input voltage
range (see Typical performance Characteristics curves).
Above V
5V.
The maximum peak switch current the device can deliver is
dependent on duty cycle. The minimum switch current value
(I
50%. For higher duty cycles, see Typical performance Char-
acteristics curves.
THERMAL CONSIDERATIONS
At higher duty cycles, the increased ON time of the FET
means the maximum output current will be determined by
power dissipation within the LM27313 FET switch. The switch
power dissipation from ON-state conduction is calculated by:
There will be some switching losses as well, so some derating
needs to be applied when calculating IC power dissipation.
MINIMUM INDUCTANCE
In some applications where the maximum load current is rel-
atively small, it may be advantageous to use the smallest
possible inductance value for cost and size savings. The con-
verter will operate in discontinuous mode in such a case.
The minimum inductance should be selected such that the
inductor (switch) current peak on each cycle does not reach
the 800 mA current limit maximum. To understand how to do
this, an example will be presented.
SW
) is guaranteed to be at least 800 mA at duty cycles below
IN
= 5V, the FET gate voltage is internally clamped to
FIGURE 4. Max. Load Current vs V
P
SW
= DC x I
SW
IND(AVG)
AND I
IN
2
x R
SW
values below 5V, since
DS(ON)
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IN
SW
in the
9
In this example, the LM27313 nominal switching frequency is
1.6 MHz, and the minimum switching frequency is
1.15 MHz. This means the maximum cycle period is the re-
ciprocal of the minimum frequency:
We will assume: V
V
Duty Cycle = ((12V + 0.3V - 5V) / (12V + 0.3V - 0.2V)) = 60.3%
Therefore, the maximum switch ON time is:
An inductor should be selected with enough inductance to
prevent the switch current from reaching 800 mA in the 0.524
µs ON time interval (see below):
The voltage across the inductor during ON time is 4.8V. Min-
imum inductance value is found by:
In this case, a 3.3 µH inductor could be used, assuming it
provided at least that much inductance up to the 800 mA cur-
rent value. This same analysis can be used to find the mini-
mum inductance for any boost application.
INDUCTOR SUPPLIERS
Some of the recommended suppliers of inductors for this
product include, but are not limited to, Sumida, Coilcraft,
Panasonic, TDK and Murata. When selecting an inductor,
make certain that the continuous current rating is high enough
to avoid saturation at peak currents. A suitable core type must
be used to minimize core (switching) losses, and wire power
losses must be considered when selecting the current rating.
SHUTDOWN PIN OPERATION
The device is turned off by pulling the shutdown pin low. If this
function is not going to be used, the pin should be tied directly
to V
must be used to V
the pin must be actively driven high and low. The SHDN pin
must not be left unterminated.
DIODE
IN
FIGURE 5. Discontinuous Design, 5V–12V Boost
. If the SHDN function will be needed, a pull-up resistor
= 0.3V. The duty cycle is:
L = 4.8V x (0.524 µs / 0.8 mA) = 3.144 µH
(60.3% x 0.870 µs) = 0.524 µs
T
ON(max)
IN
IN
(50kΩ to 100 kΩ is recommended), or
= 5V, V
L = V x (dt/dl)
= 1/1.15M = 0.870 µs
OUT
= 12V, V
SW
= 0.2V, and
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