LTM4663 datasheet(27/30 Pages) AD

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LT8722

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Rev. 0

APPLICATIONS INFORMATION

INDUCTOR SELECTION

The inductor selection determines the inductor current

ripple and loop dynamic responses. Larger inductance

results in smaller current ripple and slower transient

response as smaller inductance results in the opposite

performance. To optimize the performance, trade-offs

must be made between transient response speed, effi-

ciency and component size. Normally the inductor cur-

rent ripple is set to a value between 30% and 40% of the

maximum load current (Equation 11).

VSFB • VIN − VSFB

(

)

(VIN • fSW • ΔIL)

(11)

where ΔIL is the desired inductor current ripple in Amps.

The equivalent DC resistance (DCR) inherent in the metal

conductor of the inductor is also a critical factor for induc-

tor selection. The DCR can account for much of the power

loss in the inductor according to PLOSS = DCR • ISFB2.

Using an inductor with high DCR degrades the overall

efficiency significantly. In addition, there is a conducted

voltage drop through the inductor because of the DCR.

When the PWM amplifier is sinking current in cooling

mode, this DCR voltage drop sets the minimum voltage

of the amplifier a little higher by at least tens of millivolts.

Similarly, the maximum PWM amplifier output voltage is a

little lower by at least tens of millivolts. This voltage drop

is proportional to the value of the DCR, and reduces the

output voltage range across the TEC.

When selecting an inductor, ensure the saturation cur-

rent rating is higher than the maximum current peak to

prevent saturation. In general, ceramic multilayer induc-

tors are suitable for low current applications due to small

size and low DCR. When the noise level is critical, use a

shielded ferrite inductor to reduce the electromagnetic

interference (EMI).

SFB CAPACITOR SELECTION

The SFB capacitor determines the output voltage ripple,

transient response, as well as the loop dynamic response

of the PWM driver output. Use Equation 12 to select

the capacitor.

CSFB =

ΔIL

(8 • fSW • ΔfSFB)

(12)

where ΔVSFB is the desired maximum SFB pin

voltage ripple.

Note that the voltage caused by the product of inductor

current ripple, and the capacitor equivalent series resis-

tance (ESR) also adds to the total output voltage ripple.

Selecting a capacitor with low ESR can increase overall

regulation and efficiency performance.

Place the SFB capacitor as close to the LT8722 as possible.

LDR CAPACITOR SELECTION

To further improve systematic noise at the output of the

LT8722, additional ceramic capacitors can be added at

the LDR pin. Each additional capacitor should range from

10nF – 47nF, depending on application, and have very

low ESR and ESL characteristics. Capacitor positions are

as follows 1) between LDR – GND close to the LT8722,

2) between LDR – SFB close to the load and 3) between

LDR – GND close to the load. A lower cost, lower perfor-

mance alternative would be to place a 150nF capacitor

between LDR – SFB close to the primary SFB capacitor.

HIGH TEMPERATURE CONSIDERATIONS

The LT8722 has two over temperature monitors. If the

junction temperature exceeds ~170°C, mainly due to high

VCC regulator load current, the LT8722 will enter one ther-

mal shutdown mode, and the VCC regulator, linear driver

and PWM driver are all disabled. Otherwise, an overtem-

perature event causes the SPI register values to reset to

their default values and both drivers are disabled. Either

overtemperature event is latched in the thermal shutdown

(TSD) register bit. The TSD threshold has 15°C hysteresis

so that the LT8722 does not recover from thermal shut-

down until the on-chip temperature is below 155°C. Upon

recovery, the LT8722 will enter a new start-up sequence.

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