LT8253EUFDM datasheet(18/20 Pages) AD

18 / 20 page

LT8253/LT8253A

Rev. 0

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APPLICATIONS INFORMATION

5. Other losses. Schottky diode DB and DD are respon-

sible for conduction losses during dead time and light

load conduction periods. Inductor core loss occurs

predominately at light loads. Switch A causes reverse

recovery current loss in buck region, and switch C

causes reverse recovery current loss in boost region.

When making adjustments to improve efficiency, the

input current is the best indicator of changes in effi-

ciency. If you make a change and the input current

decreases, then the efficiency has increased. If there

is no change in the input current, then there is no

change in efficiency.

PC Board Layout Checklist

The basic PC board layout requires a dedicated ground

plane layer. Also, for high current, a multilayer board pro-

vides heat sinking for power components.

The ground plane layer should not have any traces

and it should be as close as possible to the layer with

power MOSFETs.

Place CIN, switch A, switch B and DB in one compact

area. Place COUT, switch C, switch D and DD in one

compact area.

Use immediate vias to connect the components to the

ground plane. Use several large vias for each power

component.

Use planes for VIN and VOUT to maintain good voltage

filtering and to keep power losses low.

Flood all unused areas on all layers with copper.

Flooding with copper will reduce the temperature rise

of power components. Connect the copper areas to

any DC net (VIN or GND).

Separate the signal and power grounds. All small-

signal components should return to the exposed GND

pad from the bottom, which is then tied to the power

GND close to the sources of switch B and switch C.

Place switch A and switch C as close to the controller

as possible, keeping the PGND, BG and SW traces

short.

Keep the high dV/dT SW1, SW2, BST1, BST2, TG1

and TG2 nodes away from sensitive small-signal

nodes.

The path formed by switch A, switch B, DB and the

CIN capacitor should have short leads and PCB trace

lengths. The path formed by switch C, switch D, DD

and the COUT capacitor also should have short leads

and PCB trace lengths.

The output capacitor (–) terminals should be con-

nected as close as possible to the (–) terminals of the

input capacitor.

Connect the top driver bootstrap capacitor CBST1

closely to the BST1 and SW1 pins. Connect the top

driver bootstrap capacitor CBST2 closely to the BST2

and SW2 pins.

Connect the input capacitors CIN and output capaci-

tors COUT closely to the power MOSFETs. These

capacitors carry the MOSFET AC current.

Route LSP and LSN traces together with minimum

PCB trace spacing. Avoid sense lines pass through

noisy areas, such as switch nodes. The filter capacitor

between LSP and LSN should be as close as possible

to the IC. Ensure accurate current sensing with Kelvin

connections at the RSENSE resistor. Low ESL sense

resistor is recommended.

Connect the VC pin compensation network close to

the IC, between VC and the signal ground. The capaci-

tor helps to filter the effects of PCB noise and output

voltage ripple voltage from the compensation loop.

Connect the INTVCC bypass capacitor, CINTVCC, close

to the IC, between the INTVCC and the power ground.

This capacitor carries the MOSFET drivers’ current

peaks.

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