LT8253EUFDM datasheet(13/20 Pages) AD

13 / 20 page

LT8253/LT8253A

Rev. 0

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APPLICATIONS INFORMATION

Slope compensation provides stability in constant fre-

quency current mode control by preventing subharmonic

oscillations at certain duty cycles. The minimum induc-

tance required for stability when duty cycles are larger

than 50% can be calculated as:

10 • VOUT • RSENSE

For high efficiency, choose an inductor with low core loss,

such as ferrite. Also, the inductor should have low DC

resistance to reduce the I2R losses, and must be able to

handle the peak inductor current without saturating. To

minimize radiated noise, use a shielded inductor.

RSENSE Selection and Maximum Output Current

RSENSE is chosen based on the required output current.

The duty cycle independent maximum current sense

thresholds (50mV in peak-buck and 50mV in peak-boost)

set the maximum inductor peak current in buck region,

buck-boost region, and boost region.

In boost region, the lowest maximum average load cur-

rent happens at VIN(MIN) and can be calculated as:

IOUT(MAX _BOOST) =

50mV

RSENSE

�

� IL(BOOST)

�

÷•

VIN(MIN)

VOUT

where ∆IL(BOOST) is peak-to-peak inductor ripple current

in boost region and can be calculated as:

� IL(BOOST) =

VIN(MIN) • VOUT � VIN(MIN)

(

)

f • L • VOUT

In buck region, the lowest maximum average load current

happens at VIN(MAX) and can be calculated as:

IOUT(MAX _BUCK) =

50mV

RSENSE

�

� IL(BUCK)

�

where ∆IL(BUCK) is peak-to-peak inductor ripple current in

buck region and can be calculated as:

� IL(BUCK) =

VOUT • VIN(MAX) � VOUT

(

)

f • L • VIN(MAX)

The maximum current sense RSENSE in boost region is:

RSENSE(BOOST) =

2 • 50mV • VIN(MIN)

2 • IOUT(MAX) • VOUT + � IL(BOOST) • VIN(MIN)

The maximum current sense RSENSE in buck region is

RSENSE(BUCK) =

2 • 50mV

2 • IOUT(MAX) + � IL(BUCK)

The final RSENSE value should be lower than the calculated

RSENSE in both buck and boost regions. A 20% to 30%

margin is usually recommended. Always choose a low

ESL current sense resistor.

Power MOSFET Selection

The LT8253/LT8253A require four external N-channel

power MOSFETs, two for the top switches (switches A

and D shown in Figure 1) and two for the bottom switches

(switches B and C shown in Figure 1). Important param-

eters for the power MOSFETs are the breakdown volt-

age VBR(DSS), threshold voltage VGS(TH), on-resistance

RDS(ON), reverse transfer capacitance CRSS and maximum

current IDS(MAX).

To achieve 2MHz operation, the power MOSFET selec-

tion is critical. With typical 25ns shoot-through protection

deadtime, high performance power MOSFETs with low Qg

and low RDS(ON) must be used.

Since the gate drive voltage is set by the 5V INTVCC supply,

logic-level threshold MOSFETs must be used in LT8253/

LT8253A applications. Switching four MOSFETs at higher

frequency like 2MHz, the substantial gate charge current

from INTVCC can be estimated as:

IINTVCC = f • QgA +QgB +QgC +QgD

(

)

where:

f is the switching frequency

QgA, QgB, QgC, QgD are the total gate charges of

MOSFETs A, B, C, D

Make sure the total required INTVCC current does not

exceed the INTVCC current limit in the datasheet. Typically,

MOSFETs with less than 10nC Qg are recommended.

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