SX1308

85TConverter
High Efficiency 1.2MHz 2A Step Up

85T

FEATURES

GENERAL DESCRIPTION• Integrated 80mΩ Power MOSFET• 2V to 24V Input Voltage• 1.2MHz Fixed Switching Frequency• Internal 4A Switch Current Limit• Adjustable Output Voltage• Internal Compensation• Up to 28V Output Voltage• Automatic Pulse Frequency Modulation
Mode at Light Loads• up to 96% Efficiency• Available in a 6-Pin SOT23-6 Package

The SX1 308 is a constant frequency, 6-pin SOT23
current mode step-up converter intended for small,
low power applications. The SX1308 switches at
1.2MHz and allows the use of tiny, low costcapacitors and inductors 2mm or less in height.
Internal soft-start results in small inrush current andextends battery life.
The SX1308 features automatic shifting to pulse
frequency modulation mode at light loads. The
SX1308 includes under-voltage lockout, current
limiting, and thermal overload protection to prevent
damage in the event of an output overload. The
SX1308 is available in a small 6-pin SOT-23
package.

APPLICATIONS• Battery-Powered Equipment• Set-Top Boxed• LCD Bais Supply• DSL and Cable Modems and Routers• Networking cards powered from PCIor PCI express slots

TYPICAL APPLICATION

Efficiency (%)

Efficiency

Iout (mA)

Figure 1. Basic Application Circuit

Figure 2. Efficiency Curve

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SX1308

ABSOLUTE MAXIMUM RATINGS
IN, EN voltages ………..….…… -0.3V to 26V
Operating Temperature….... -40°C to +85°C
FB Voltages ................................-0.3V to 6V
Junction Temperature ………………...160°C

SW Voltage ……………………..-0.3V to 30V
Storage Temperature Range -65°C to 150°CPeak SW Sink and Source Current ………4A
Lead Temperature (Soldering, 10s) ...+300°C

PACKAGE/ORDER INFORMATION
SW

16NC

GND

25INFB

34EN

SOT23-5
SOT23-6(MT3540)(SX1308)

PIN DESCRIPTION
PIN

NAME

FUNCTION

1SW

Power Switch Output. SW is the drain of the internal MOSFET switch. Connect thepower inductor and output rectifier to SW. SW can swing between GND and 28V.

2GND

Ground Pin

3FB

Feedback Input. The FB voltage is 0.6V. Connect a resistor divider to FB.

4ENRegulator On/Off Control Input. A high input at EN turns on the converter, and alow input turns it off. When not used, connect EN to the input supply for automaticstartup.

5IN

6NC

Input Supply Pin. Must be locally bypassed.
NC

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SX1308

ELECTRICAL CHARACTERISTICS(VIN=VEN=5V, TA = 25°C, unless otherwise noted.)Parameter

Conditions

Operating Input Voltage

MIN

TYP

2

Under Voltage Lockout
Under Voltage Lockout Hysteresis

MAXunit

24

V

1.98

V100mV

Current (Shutdown)

VEN= 0V

0.1

1µA

Quiescent Current (PFM)

VFB=0.7V，No switch

100200µA

Quiescent Current (PWM)

VFB=0.5V，switch

1.6

2.2mA

Switching Frequency
Maximum Duty Cycle

1.2
VFB = 0V

EN Input High Voltage

90%

1.5V

EN Input Low Voltage
FB Voltage
FB Input Bias Current

VFB = 0.6V

0.588

0.6-50-10

SW On Resistance (1)80

SW Current Limit (1)VIN= 5V, Duty cycle=50%

SW Leakage

VSW = 20V

MHz

Thermal Shutdown

0.4

V

0.612

VnA

1504A
1155mΩ
μA℃

Note:1) Guaranteed by design, not tested.

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OPERATION
The SX1308 uses a fixed frequency, peak current
mode boost regulator architecture to regulate
voltage at the feedback pin. The operation of theSX1308 can be understood by referring to the block
diagram of Figure 3. At the start of each oscillator
cycle the MOSFET is turned on through the control
circuitry. To prevent sub-harmonic oscillations at
duty cycles greater than 50 percent, a stabilizing
ramp is added to the output of the current sense
amplifier and the result is fed into the negative inputof the PWM comparator. When this voltage equals

The output voltage of the error amplifier the power
MOSFET is turned off. The voltage at the output of
the error amplifier is an amplified version of thedifference between the 0.6V bandgap reference
voltage and the feedback voltage. In this way thepeak current level keeps the output in regulation. If
the feedback voltage starts to drop, the output of theerror amplifier increases. These results in morecurrent to flow through the power MOSFET, thusincreasing the power delivered to the output. TheSX1308 has internal soft start to limit the amount of
input current at startup and to also limit the amountof overshoot on the output.

Figure 3. Functional Block Diagram

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SX1308

TYPICAL OPERATING CHARACTERISTICS
Efficiency Curve

Efficiency

Current limit(A)Efficiency Curve

Iout(mA)

line Regulation

Load regualation

Vout(V)

Vout(V)

Iout (mA)

Vin (V)

Iout (mA)

Freq VS Vin

Efficiency VS Vin

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SX1308

APPLICATION INFORMATION
Setting the Output Voltage

Layout Consideration

The internal reference VREF is 0.6V (Typical).Theoutput voltage is divided by a resistor divider,R1 andR2 to the FB pin. The output voltage is given by

For best performance of the SX1308, the following
guidelines must be strictly followed.
 Input and Output capacitors should be placed
close to the IC and connected to ground plane
to reduce noise coupling.
 The GND should be connected to a strong
ground plane for heat sinking and noise
protection.
 Keep the main current traces as possible asshort and wide.
 SW node of DC-DC converter is with highfrequency voltage swing. It should be kept at asmall area.
 Place the feedback components as close as
possible to the IC and keep away from thenoisy devices.

VOUT = VREF × (1 +R1)
R2Inductor Selection
The recommended values of inductor are 4.7 to22μH. Small size and better efficiency are the major
concerns for portable device, such as SX1308 used
for mobile phone. The inductor should have low coreloss at 1.2MHz and low DCR for better efficiency. To
avoid inductor saturation current rating should beconsidered.

Capacitor Selection
Input and output ceramic capacitors of 22μF arerecommended for SX1308 applications. For better
voltage filtering, ceramic capacitors with low ESR
are recommended. X5R and X7R types are suitablebecause of their wider voltage and temperature
ranges.

Diode Selection
Schottky diode is a good choice for SX1308
because of its low forward voltage drop and fast
reverses recovery. Using Schottky diode can getbetter efficiency. The high speed rectification is alsoa good characteristic of Schottky diode for highswitching frequency. Current rating of the diode mustmeet the root mean square of the peak current andoutput average current multiplication as following :I D ( RMS ) ≈ I OUT × I PEAK
The diode’ s reverse breakdown voltage should belarger than the output voltage.

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PACKAGE DESCRIPTIONTOP VIEW

RECOMMENDED LAND PATTERNFRONT VIEW

SIDE VIEW

DETAIL “A”

Figure 4. TSOT23-6/SOT23-6 Physical Dimensions

NOTE:1)ALL DIMENSIONS ARE IN MILLIMETERS.
2) PACKAGE LENGTH DOES NOT INCLUDE MOLD FLASH,PROTRUSION OR GATE BURR.
3) PACKAGE WIDTH DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION.
4) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.10 MILLIMETERS MAX.
5) DRAWING CONFORMS TO JEDEC MO-193, VARIATION AB.
6) DRAWING IS NOT TO SCALE.
7) PIN 1 IS LOWER LEFT PIN WHEN READING TOP MARK FROM LEFT TO RIGHT, (SEE EXAMPLE TOP MARK)

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