LM5122MHXSLASHNOPB datasheet(37/49 Pages) TI1

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VIN

LM5122

VIN

RVIN

CVIN

OUT

RIPPLE _ MAX(CIN)

24V

0.09

32 L

32 10 +

)

N+]

u u

OUT

RIPPLE _ MAX(COUT)

ESR

IN(MIN)

OUT

4.5A

60m

0.252V

4 C

4 3 330 )

N+]

24V

u u

LM5122, LM5122-Q1

www.ti.com

SNVS954G – FEBRUARY 2013 – REVISED MAY 2016

Product Folder Links: LM5122 LM5122-Q1

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Assuming 60 m

Ω of ESR per an output capacitor, the output voltage ripple at the minimum input voltage is

calculated as follows:

(33)

In practice, four 10 µF ceramic capacitors are additionally placed earlier than the bulk aluminum capacitors to

reduce the output voltage ripple and split the output ripple current.

Due to the inherent path from input to output, unlimited inrush current can flow when the input voltage rises

quickly and charges the output capacitor. The slew rate of input voltage rising should be controlled by a hot-swap

or by starting the input power supply softly for the inrush current not to damage the inductor, sense resistor or

high-side N-channel MOSFET switch.

8.2.2.8 Input Capacitor CIN

The input capacitors smooth the input voltage ripple. Assuming high quality ceramic capacitors are used for the

input capacitors, the maximum input voltage ripple which happens when the input voltage is half of the output

voltage can be calculated as follows:

(34)

The value of input capacitor is also a function of source impedance, the impedance of source power supply. The

more input capacitor will be required to prevent a chatter condition upon power up if the impedance of source

power supply is not enough low.

8.2.2.9 VIN Filter RVIN, CVIN

An R-C filter (RVIN, CVIN) on VIN pin is optional. It is not required if CIN capacitors are high quality ceramic

capacitors and placed physically close to the device. The filter helps to prevent faults caused by high frequency

switching noise injection into the VIN pin. A 0.47

μF ceramic capacitor is used this example. 3 Ω of RVIN and 0.47

µF of CVIN are normal recommendations. A larger filter with 2.2 µ~4.7 µF CVIN is recommended when the input

voltage is lower than 8 V or the required duty cycle is close to the maximum duty cycle limit.

Figure 41. VIN Filter

8.2.2.10 Bootstrap Capacitor CBST and Boost Diode DBST

The bootstrap capacitor between the BST and SW pin supplies the gate current to charge the high-side N-

channel MOSFET device gate during each cycle’s turn-on and also supplies recovery charge for the bootstrap

diode. These current peaks can be several amperes. The recommended value of the bootstrap capacitor is 0.1

μF. CBST should be a good quality, low ESR, ceramic capacitor located at the pins of the device to minimize

potentially damaging voltage transients caused by trace inductance. The minimum value for the bootstrap

capacitor is calculated as follows:

(35)

Where QG is the high-side N-channel MOSFET gate charge and ΔVBST is the tolerable voltage droop on CBST,

which is typically less than 5% of VCC or 0.15 V, conservatively. In this example, the value of the BST capacitor

(CBST) is 0.1 µF.

LM5122MHX/NOPB Datasheet(PDF) 37 Page - Texas Instruments

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