LT8640 datasheet(21/28 Pages) AD

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LT8640A

Rev. 0

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APPLICATIONS INFORMATION

improve efficiency the internal LDO can also draw cur-

rent from the BIAS pin when the BIAS pin is at 3.1V or

higher. Typically the BIAS pin can be tied to the output of

the LT8640A, or can be tied to an external supply of 3.3V

or above. If BIAS is connected to a supply other than

VOUT, be sure to bypass with a local ceramic capacitor. If

the BIAS pin is below 3.0V, the internal LDO will consume

current from VIN. Applications with high input voltage and

high switching frequency where the internal LDO pulls

current from VIN will increase die temperature because

of the higher power dissipation across the LDO. Do not

connect an external load to the INTVCC pin.

Output Voltage Tracking and Soft-Start

The LT8640A allows the user to program its output voltage

ramp rate by means of the TR/SS pin. An internal 1.9µA

pulls up the TR/SS pin to INTVCC. Putting an external

capacitor on TR/SS enables soft starting the output to

prevent current surge on the input supply. During the soft-

start ramp the output voltage will proportionally track the

TR/SS pin voltage. For output tracking applications, TR/

SS can be externally driven by another voltage source.

From 0V to 0.97V, the TR/SS voltage will override the

internal 0.97V reference input to the error amplifier, thus

regulating the FB pin voltage to that of TR/SS pin. In the

LT8640A-3.3 fixed output voltage option, the output volt-

age will track the TR/SS pin voltage based on the 3.3/0.97

factor set by the internal feedback resistor divider. When

TR/SS is above 0.97V, tracking is disabled and the feed-

back voltage will regulate to the internal reference volt-

age. The TR/SS pin may be left floating if the function is

not needed.

An active pull-down circuit is connected to the TR/SS pin

which will discharge the external soft-start capacitor in

the case of fault conditions and restart the ramp when the

faults are cleared. Fault conditions that clear the soft-start

capacitor are the EN/UV pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Output Power Good

When the LT8640A’s output voltage is within the ±8%

window of the regulation point, the output voltage is

considered good and the open-drain PG pin goes high

impedance and is typically pulled high with an external

resistor. Otherwise, the internal pull-down device will pull

the PG pin low. To prevent glitching both the upper and

lower thresholds include 0.3% of hysteresis. PG is valid

when VIN is above 3.4V.

The PG pin is also actively pulled low during several fault

conditions: EN/UV pin is below 1V, INTVCC has fallen too

low, VIN is too low, or thermal shutdown.

Shorted and Reversed Input Protection

The LT8640A will tolerate a shorted output. Several fea-

tures are used for protection during output short-circuit

and brownout conditions. The first is the switching

frequency will be folded back while the output is lower

than the set point to maintain inductor current control.

Figure 5. Reverse VIN Protection

VIN

LT8640A

EN/UV

8640a F05

GND

Second, the bottom switch current is monitored such that

if inductor current is beyond safe levels switching of the

top switch will be delayed until such time as the inductor

current falls to safe levels.

Frequency foldback behavior depends on the state of the

SYNC pin: If the SYNC pin is low the switching frequency

will slow while the output voltage is lower than the pro-

grammed level. If the SYNC pin is connected to a clock

source, floated or tied high, the LT8640A will stay at the

programmed frequency without foldback and only slow

switching if the inductor current exceeds safe levels.

There is another situation to consider in systems where the

output will be held high when the input to the LT8640A is

absent. This may occur in battery charging applications or

in battery-backup systems where a battery or some other

supply is diode ORed with the LT8640A’s output. If the VIN

pin is allowed to float and the EN pin is held high (either

by a logic signal or because it is tied to VIN), then the

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