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LT8362

8362fa

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Figure 3. Soft-Start Waveforms

APPLICATIONS INFORMATION

FAULT PROTECTION

An inductor overcurrent fault (> 3.75A) and/or INTVCC

undervoltage (INTVCC < 2.5V) and/or thermal lockout

(TJ > 170°C) will immediately prevent switching, will

reset the SS pin and will pull down VC. Once all faults are

removed, the LT8362 will soft-start VC and hence inductor

peak current.

FREQUENCY FOLDBACK

During start-up or fault conditions in which VOUT is very

low, extremely small duty cycles may be required to main-

tain control of inductor peak current. The minimum on-

time limitation of the power switch might prevent these

low duty cycles from being achievable. In this scenario

inductor current rise will exceed inductor current fall

during each cycle, causing inductor current to “walk up”

beyond the switch current limit. The LT8362 provides

protection from this by folding back switching frequency

whenever FBX or SS pins are close to GND (low VOUT

levels or start-up). This frequency foldback provides a

larger switch-off time, allowing inductor current to fall

enough each cycle (see Normalized Switching Frequency

vs FBX Voltage in the Typical Performance Characteristics

section).

THERMAL LOCKOUT

If the LT8362 die temperature reaches 170°C (typical),

the part will stop switching and go into thermal lockout.

When the die temperature has dropped by 5°C (nominal),

the part will resume switching with a soft-started inductor

peak current.

COMPENSATION

Loop compensation determines the stability and transient

performance. The LT8362 uses current mode control to

regulate the output which simplifies loop compensation.

The optimum values depend on the converter topology, the

component values and the operating conditions (including

the input voltage, load current, etc.). To compensate the

feedback loop of the LT8362, a series resistor-capacitor

network is usually connected from the VC pin to GND.

The Block Diagram shows the typical VC compensation

network. For most applications, the capacitor should be

in the range of 100pF to 10nF, and the resistor should

be in the range of 5k to 100k. A small capacitor is often

connected in parallel with the RC compensation network

to attenuate the VC voltage ripple induced from the out-

put voltage ripple through the internal error amplifier. The

parallel capacitor usually ranges in value from 2.2pF to

22pF. A practical approach to designing the compensa-

tion network is to start with one of the circuits in this data

sheet that is similar to your application, and tune the com-

pensation network to optimize the performance. Stability

should then be checked across all operating conditions,

including load current, input voltage and temperature.

Application Note 76 is a good reference.

THERMAL CONSIDERATIONS

Care should be taken in the layout of the PCB to ensure

good heat sinking of the LT8362. Both packages have an

exposed pad underneath the IC which is the best path

for heat out of the package. The exposed pad should be

soldered to a continuous copper ground plane under the

device to reduce die temperature and increase the power

capability of the LT8362. The ground plane should be

connected to large copper layers to spread heat dissi-

pated by the LT8362. Power dissipation within the LT8362

(PDISS_LT8362) can be estimated by subtracting the induc-

tor and Schottky diode power losses from the total power

losses calculated in an efficiency measurement. The junc-

tion temperature of LT8362 can then be estimated by:

TJ(LT8362) = TA + θJA • PDISS_LT8362

500µs/DIV

VOUT

20V/DIV

1A/DIV

8362 F03

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