LT8604 datasheet(15/20 Pages) AD

15 / 20 page

LT8604

Rev. 0

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ground with a minimum of 1μF ceramic capacitor. Good

bypassing is necessary to supply the high transient cur-

rents required by the power MOSFET gate drivers. To

improve efficiency, the internal LDO can also draw cur-

rent from the BIAS pin when the BIAS pin is at 3.2V or

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

the LT8604 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 LT8604 allows the user to program its output voltage

ramp rate by means of the TR/SS pin. An internal 2μA

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 0.778V, the TR/SS voltage will override the

internal 0.778V reference input to the error amplifier, thus

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

TR/SS is above 0.778V, tracking is disabled and the feed-

back voltage will regulate to the internal reference voltage.

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 LT8604’s output voltage is within the ±7.5%

window of the regulation point, which is a VFB voltage in

the range of 0.720V to 0.836V (typical), 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 drain pull-down device

will pull the PG pin low. To prevent glitching both the

upper and lower thresholds include 0.5% of hysteresis.

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 LT8604 will tolerate a shorted output. Several features

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. 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. This allows for tailoring the LT8604

to individual applications and limiting thermal dissipation

during short circuit conditions.

There is another situation to consider in systems where

the output will be held high when the input to the LT8604

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 LT8604’s output. If

the VIN pin is allowed to float and the EN/UV pin is held

high (either by a logic signal or because it is tied to VIN),

then the LT8604’s internal circuitry will pull its quiescent

current through its SW pin. This is acceptable if the sys-

tem can tolerate several μA in this state. If the EN/UV pin

is grounded the SW pin current will drop to near 0.7µA.

However, if the VIN pin is grounded while the output is held

high, regardless of EN/UV, parasitic body diodes inside

the LT8604 can pull current from the output through the

SW pin and the VIN pin. Figure 3 shows a connection of

the VIN and EN/UV pins that will allow the LT8604 to run

only when the input voltage is present and that protects

against a shorted or reversed input.

APPLICATIONS INFORMATION

VIN

LT8604

GND

8604 F03

EN/UV

Figure 3. Reverse VIN Protection

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