LT8331 datasheet(12/34 Pages) AD

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LT8357

Rev. 0

For more information www.analog.com

The front page shows a typical LT8357 application circuit.

This Applications Information section serves as a guide-

line for selecting external components for typical applica-

tions. The examples and equations in this section assume

continuous conduction mode unless otherwise specified.

Switching Frequency Selection

The LT8357 uses a constant frequency control scheme

between 100kHz and 2MHz. Selection of the switching

frequency is a trade-off between efficiency and compo-

nent size. Low frequency operation improves efficiency

by reducing MOSFET switching losses, but requires larger

inductor and capacitor values. For high power applica-

tions, consider operating at lower frequencies to minimize

MOSFET heating from switching losses. For low power

applications, consider operating at higher frequencies to

minimize the total solution size.

In addition, the specific application also plays an important

role in switching frequency selection. In a noise-sensitive

system, the switching frequency is usually selected to keep

the switching noise out of a sensitive frequency band.

Switching Frequency Setting

The switching frequency of the LT8357 is set by the inter-

nal oscillator. With frequency synchronization disabled,

the switching frequency can be set by placing a resistor

from the RT pin to ground. Table 1 shows RT resistor

values for common switching frequencies.

Table 1. Switching Frequency vs RT Value (1% Resistor)

fOSC (kHz)

RT (kΩ)

100

357

200

174

350

95.3

400

82.5

600

53.6

800

40.2

1000

31.6

1200

26.1

1400

22.1

1600

19.1

1800

16.9

2000

15.0

APPLICATIONS INFORMATION

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome for

applications where electromagnetic interference (EMI) is a

concern. To improve EMI performance, the LT8357 imple-

ments a triangle spread spectrum frequency modulation

scheme. When the SYNC/MODE pin is tied to INTVCC or

a 100kΩ resistor is placed from the SYNC/MODE pin

to ground, the LT8357 spreads its switching frequency

19% above the internal oscillator frequency. Figure 2

and Figure 3 show the noise spectrum of the front page

application with spread spectrum frequency modulation

enabled and disabled.

Split Gate Drive

It is critical to optimize gate drive for EMI performance in

switching regulators by adding an external gate resistor,

but there is a trade-off between power efficiency and EMI.

Slow gate drive with pull-up gate resistor improves EMI

but at the cost of reduced efficiency. Strong pull-down

gate drive is always required because the fast slew rate

of SW node may pull up the gate of the power MOSFET

and falsely turn it on. A single gate drive can only have a

single gate resistor that cannot optimize both efficiency

and EMI at the same time. In the LT8357, a split gate drive

is implemented, which allows using a large pull-up resis-

tor and low pull-down resistor to achieve an optimized

efficiency and EMI performance.

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