LT8697 datasheet(20/40 Pages) AD

20 / 40 page

Additionally, the LT8698S can sink current from the out-

put when in forced continuous mode (FCM). This feature

improves the step response for a load step from high to

low. Cable drop compensation adds voltage to the output

to compensate for voltage drop across the cable resis-

tance at high load. Since most DC/DC convertors can only

source current, a load step from high to near zero current

leaves the output voltage high and out of regulation. The

LT8698S fixes this problem by allowing the regulator to

LT8698S/LT8698S-1

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

sink current from the output when USB5V is too high

using FCM. Figure 5 shows the output voltage of the front

page application circuit with both FCM and PSK modes in

response to a fast, 1.3A load step through a 3m cable. The

load step response from high current to zero in PSK mode

is extremely slow and is limited by the OUT/ISP, BUS/ISN

and USB5V pin bias currents. However, with FCM enabled,

the output slews quickly back into regulation.

Figure 5. Load Step Response with and without

Forced Continuous Mode

20µs/DIV

VLOAD w/ FCM

500mV/DIV

VLOAD w/o FCM

500mV/DIV

IBUS

1A/DIV

8698S F05

VBUS

CBUS

100µF

LONG CABLE

8698S F04a

PROBE

POINT

PROBE

GROUND

POINT

VBUS

CBUS

100µF

LONG CABLE

8698S F04b

PROBE

POINT

PROBE

POINT

(a) Incorrect Remote Output Probing

(b) Correct Remote Output Probing

100µs/DIV

VLOAD Probed

Correctly

500mV/DIV

VLOAD Probed

Incorrectly

500mV/DIV

IBUS

1A/DIV

8698S F04c

Figure 4. Probing a Remote Output Correctly

Lastly, the LT8698S positive switch peak current and

negative valley current limits exceed the positive output

current limit of 2.65A. This large current range allows the

LT8698S to slew the output capacitor quickly for cable

drop compensation during a transient load step.

Output Current Limit

In addition to regulating the output voltage, the LT8698S

limits the average output current with a current regulation

loop. Output current regulation limits power dissipation

in the case of a fault on the USB cable or of the device

plugged into the socket. The LT8698S measures the volt-

age drop across the external sense resistor RSEN using the

OUT/ISP and BUS/ISN pins. This resistor should be con-

nected on the load side of the output capacitor COUT, in

series with the load. The LT8698S control loop modulates

the cycle-by-cycle switch current limit such that the aver-

age voltage across the OUT/ISP to BUS/ISN pins does not

exceed its regulation point. When load current exceeds

the output current limit and current regulation is active,

the output voltage will drop below its regulation point.

The LT8698S also includes latch-off and auto-retry func-

tionality in the output current regulation loop. If the output

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