LT8607 datasheet(16/24 Pages) AD

16 / 24 page

LT8607/LT8607B

Rev. D

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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 LT8607 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

LT8607. 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 cur-

rent for the LT8607’s circuitry and must be bypassed to

ground with a minimum of 1µF ceramic capacitor. Good

bypassing 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.

Output Voltage Tracking and Soft-Start (MSOP Only)

The LT8607 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

APPLICATIONS INFORMATION

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 regulat-

ing the FB pin voltage to that of TR/SS pin.

In the LT8607-5 fixed output option, the output voltage

will track the TR/SS pin to 6.43 times the TR/SS voltage,

a voltage based on a factor set by the internal feedback

resistor divider. When TR/SS is above 0.778V, tracking

is disabled and the feedback 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 volt-

age falling too low, or thermal shutdown. The LT8607 and

LT8607B DFN does not have the TR/SS pin or functionality.

Output Power Good

When the LT8607’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 (MSOP Only)

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 LT8607 oscillator to an external frequency

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).

The LT8607 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will pulse skip to maintain regulation. The LT8607

may be synchronized over a 200kHz to 2.2MHz range. The

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