LM5122MHXSLASHNOPB datasheet(39/49 Pages) TI1

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RR(HS)

OUT

[W]

DT(HS)

DLH

DHL

V x I

x (t

) x f

[W]

OUT

COND(HS)

DS _ ON(HS)

OUT

(1 D) I

1.3

1.3[W]

· §

¸ ¨

SW(LS)

OUT

0.5 V

t ) f

[W]

LM5122, LM5122-Q1

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SNVS954G – FEBRUARY 2013 – REVISED MAY 2016

Product Folder Links: LM5122 LM5122-Q1

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Where, D is the duty cycle and the factor of 1.3 accounts for the increase in the N-channel MOSFET device on-

resistance due to heating. Alternatively, the factor of 1.3 can be eliminated and the high temperature on-

resistance of the N-channel MOSFET device can be estimated using the RDS(ON) vs temperature curves in the N-

channel MOSFET datasheet.

Switching loss occurs during the brief transition period as the low-side N-channel MOSFET device turns on and

off. During the transition period both current and voltage are present in the channel of the N-channel MOSFET

device. The low-side switching loss is approximately calculated as follows:

(41)

tR and tF are the rise and fall times of the low-side N-channel MOSFET device. The rise and fall times are usually

mentioned in the N-channel MOSFET datasheet or can be empirically observed with an oscilloscope.

An additional Schottky diode can be placed in parallel with the low-side N-channel MOSFET switch, with short

connections to the source and drain in order to minimize negative voltage spikes at the SW node.

8.2.2.16 High-Side Power Switch QH and Additional Parallel Schottky Diode

Losses in the high-side N-channel MOSFET device can be separated into conduction loss, dead-time loss and

reverse recovery loss. Switching loss is calculated for the low-side N-channel MOSFET device only. Switching

loss in the high-side N-channel MOSFET device is negligible because the body diode of the high-side N-channel

MOSFET device turns on before and after the high-side N-channel MOSFET device switches.

High-side conduction loss is approximately calculated as follows:

(42)

Dead-time loss is approximately calculated as follows:

where

•

VD is the forward voltage drop of the high-side NMOS body diode.

(43)

Reverse recovery characteristics of the high-side N-channel MOSFET switch strongly affect efficiency, especially

when the output voltage is high. Small reverse recovery charge helps to increase the efficiency while also

minimizes switching noise.

Reverse recovery loss is approximately calculated as follows:

(44)

where

•

QRR is the reverse recovery charge of the high-side N-channel MOSFET body diode.

(45)

An additional Schottky diode can be placed in parallel with the high-side switch to improve efficiency. Usually, the

power rating of this parallel Schottky diode can be less than the high-side switch’s because the diode conducts

only during dead-times. The power rating of the parallel diode should be equivalent or higher than high-side

switch’s if bypass operation is required, hiccup mode operation is required or any load exists before switching.

8.2.2.17 Snubber Components

A resistor-capacitor snubber network across the high-side N-channel MOSFET device reduces ringing and

spikes at the switching node. Excessive ringing and spikes can cause erratic operation and can couple noise to

the output voltage. Selecting the values for the snubber is best accomplished through empirical methods. First,

make sure the lead lengths for the snubber connections are very short. Start with a resistor value between 5 and

Ω. Increasing the value of the snubber capacitor results more damping, but this also results higher snubber

losses. Select a minimum value for the snubber capacitor that provides adequate damping of the spikes on the

switch waveform at heavy load. A snubber may not be necessary with an optimized layout.

8.2.2.18 Loop Compensation Components CCOMP, RCOMP, CHF

RCOMP, CCOMP and CHF configure the error amplifier gain and phase characteristics to produce a stable voltage

loop. For a quick start, follow the following 4 steps:

LM5122MHX/NOPB Datasheet(PDF) 39 Page - Texas Instruments

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