LT8607 datasheet(16/24 Pages) AD

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LT8609S

Rev. D

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can be tied to VIN if the shutdown feature is not used, or

tied to a logic level if shutdown control is required.

Adding a resistor divider from VIN to EN programs the

LT8609S to regulate the output only when VIN is above

a desired voltage (see Block Diagram). Typically, this

threshold, VIN(EN), is used in situations where the input

supply is current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, so source current increases as source

voltage drops. This looks like a negative resistance load

to the source and can cause the source to current limit or

latch low under low source voltage conditions. The VIN(EN)

threshold prevents the regulator from operating at source

voltages where the problems might occur. This threshold

can be adjusted by setting the values R3 and R4 such that

they satisfy the following equation:

VIN(EN) =

⎛

⎝

⎜

⎞

⎠

⎟•1V

where the LT8609S will remain off until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching

will not stop until the input falls slightly below VIN(EN).

When in Burst Mode operation for light-load currents,

the current through the VIN(EN) resistor network can eas-

ily be greater than the supply current consumed by the

LT8609S. Therefore, the VIN(EN) resistors should be large

to minimize their effect on efficiency at low loads.

INTVCC Regulator

An internal low dropout (LDO) regulator produces the 3.5V

supply from VIN that powers the drivers and the internal

bias circuitry. The INTVCC can supply enough current for

the LT8609S’s circuitry and must be bypassed to ground

with a minimum of 1μF ceramic capacitor. Good bypass-

ing is necessary to supply the high transient currents

required by the power MOSFET gate drivers. Applications

with high input voltage and high switching frequency will

increase die temperature because of the higher power

dissipation across the LDO. Do not connect an external

load to the INTVCC pin.

APPLICATIONS INFORMATION

Output Voltage Tracking and Soft-Start

The LT8609S 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 0.774V, the TR/SS voltage will override the

internal 0.774V reference input to the error amplifier, thus

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

TR/SS is above 0.774V, 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 LT8609S’s output voltage is within the ±8.5%

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

the range of 0.716V to 0.849V (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.

Synchronization

To select low ripple Burst Mode operation, tie the SYNC

pin below 0.4V (this can be ground or a logic low output).

To synchronize the LT8609S oscillator to an external fre-

quency connect a square wave (with 20% to 80% duty

cycle) to the SYNC pin. The square wave amplitude should

have valleys that are below 0.9V and peaks above 2.7V

(up to 5V).

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