LT8640 datasheet(23/28 Pages) AD

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LT8638S

Rev. 0

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APPLICATIONS INFORMATION

Output Power Good

When the LT8638S device’s output voltage is within the

±7.75% 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 0.4% of hysteresis. PG is valid

when VIN is above 2.8V.

The PG pin is also actively pulled low during several fault

conditions: EN/UV pin is below 0.98V, INTVCC has fallen

too low, VIN is too low, or thermal shutdown.

Shorted and Reversed Input Protection

The LT8638S will tolerate a shorted output. Several fea-

tures are used for protection during output short-cir-

cuit 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.

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 LT8638S 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 LT8638S

is absent. This may occur in battery charging applica-

tions or in battery-backup systems where a battery or

some other supply is diode ORed with the LT8638S

device’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 LT8638S device’s internal

circuitry will pull its quiescent current through its SW

pin. This is acceptable if the system can tolerate several

µA in this state. If the EN pin is grounded the SW pin

current will drop to near 6µA. However, if the VIN pin is

grounded while the output is held high, regardless of

EN, parasitic body diodes inside the LT8638S can pull

current from the output through the SW pin and the

VIN pin. Figure 7 shows a connection of the VIN and

EN/UV pins that will allow the LT8638S to run only when

the input voltage is present and that protects against a

shorted or reversed input.

VIN

LT8638S

EN/UV

8638S F07

GND

Figure 7. Reverse VIN Protection

Thermal Considerations and Peak Output Current

For higher ambient temperatures, care should be taken in

the layout of the PCB to ensure good heat sinking of the

LT8638S. The ground pins on the bottom of the package

should be soldered to a ground plane. This ground should

be tied to large copper layers below with thermal vias;

these layers will spread heat dissipated by the LT8638S.

Placing additional vias can reduce thermal resistance

further. The maximum load current should be derated

as the ambient temperature approaches the maximum

junction rating. Power dissipation within the LT8638S

can be estimated by calculating the total power loss from

an efficiency measurement and subtracting the inductor

loss. The die temperature is calculated by multiplying the

LT8638S power dissipation by the thermal resistance

from junction to ambient.

The internal overtemperature protection monitors the

junction temperature of the LT8638S. If the junction

temperature reaches approximately 175°C, the LT8638S

will stop switching and indicate a fault condition until the

temperature drops about 10°C cooler.

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