LM5122ZPWPR datasheet(35/51 Pages) TI1

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PEAK

OUT

24V 4.5A

8.7V

13.5 A

8.7V

10 +

N+]

OUT

12V

10.7 +

108W

250 kHz

24V

12V

UV2

UV1

IN(STARTUP)

1.2V R

1.2V 50 k

8 k

1.2V

8.7V

1.2V

HYS

UV2

HYS

0.5 V

50 k

10 A

9 10

36.0 k

0 kHz

LM5122, LM5122-Q1

www.ti.com

SNVS954G – FEBRUARY 2013 – REVISED MAY 2016

Product Folder Links: LM5122 LM5122-Q1

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Typical Application (continued)

8.2.1 Design Requirements

DESIGN PARAMETERS

VALUE

Output Voltage (VOUT)

24 V

Full Load Current (IOUT)

4.5 A

Output Power

108 W

Minimum Input Voltage (VIN(MIN))

9 V

Typical Input Voltage (VIN(TYP))

12 V

Maximum Input Voltage (VIN(MAX))

20 V

Switching Frequency (fSW)

250 kHz

8.2.2 Detailed Design Procedure

8.2.2.1 Timing Resistor RT

Generally, higher frequency applications are smaller but have higher losses. Operation at 250 kHz is selected for

this example as a reasonable compromise between small size and high-efficiency. The value of RT for 250 kHz

switching frequency is calculated as follows:

(23)

A standard value of 36.5 k

Ω is chosen for RT.

8.2.2.2 UVLO Divider RUV2, RUV1

The desired startup voltage and the hysteresis are set by the voltage divider RUV2, RUV1. The UVLO shutdown

voltage should be high enough to enhance the low-side N-channel MOSFET switch fully. For this design, the

startup voltage is set to 8.7 V which is 0.3 V below VIN(MIN). VHYS is set to 0.5 V. This results 8.2 V of

VIN(SHUTDOWN). The values of RUV2, RUV1 are calculated as follows:

(24)

(25)

A standard value of 49.9 k

Ω is selected for RUV2. RUV1 is selected to be a standard value of 8.06 kΩ.

8.2.2.3 Input Inductor LIN

The inductor ripple current is typically set between 20% and 40% of the full load current, known as a good

compromise between core loss and copper loss of the inductor. Higher ripple current allows for a smaller inductor

size, but places more of a burden on the output capacitor to smooth the ripple voltage on the output. For this

example, a ripple ratio (RR) of 0.25, 25% of the input current was chosen. Knowing the switching frequency and

the typical output voltage, the inductor value can be calculated as follows:

(26)

The closest standard value of 10

μH was chosen for LIN.

The saturation current rating of inductor should be greater than the peak inductor current, which is calculated at

the minimum input voltage and full load. 8.7 V startup voltage is used conservatively.

(27)

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