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LTC1760

1760fa

Figure 6. PowerPath Driver Equivalent Circuit

–

GB1I

GB1O

FROM

BATTERY

BAT1

SCP

25mV

20mV

OFF

1760 F06

EAP

SWP

TO LOAD

RSC

OPERATION

8.1 Autonomous PowerPath Switching

The LOPWR comparator monitors the voltage at the

load through the resistor divider from pin SCN. If LTC

(POWER_OFF) is low and the LOPWR comparator trips,

then all of the switches are turned on (3-Diode mode)

by the Autonomous PowerPath Controller to ensure that

the system is powered from the source with the highest

voltage. The Autonomous PowerPath Controller waits

approximately 1 second, to allow power to stabilize, and

then reverts back to the PowerPath switch configuration

requested by the PowerPath Management Algorithm. A

power fail counter is incremented to indicate that a failure

has occurred. If the power fail counter equals a value of 3,

then the the Autonomous PowerPath Controller sets the

switches to 3-Diode mode and BatterySystem-

StateCont(POWER_NOT_GOOD) will be set, provided

the LOPWR comparator is still detecting a low power

event. This is a three-strikes-and-you’re-out process

which is intended to debounce the POWER_NOT_GOOD

indicator. The power fail counter is reset when battery or

AC presence change.

8.2 Short-Circuit Protection

Short-circuit protection operates in both a current mode

and a voltage mode. If the voltage between SCP and

SCN exceeds the short-circuit comparator threshold

VTSC for more than 15ms, then all of the PowerPath

switches are turned off and BatterySystemState-Cont

(POWER_NOT_GOOD) is set. Similarly, if the voltage

at SCN falls below 3V for more than 15ms, then all of

the PowerPath switches are turned off and POWER_

NOT_GOOD is set high. The POWER_NOT_GOOD bit is

reset by removing all power sources and allowing the

voltage at VPLUS to fall below the UVLO threshold. If

the POWER_NOT_GOOD bit is set, charging is disabled

until VPLUS exceeds the UVLO threshold and the Charger

Algorithm allows charging to resume.

When a hard short-circuit occurs, it might pull all of the

power sources down to near 0V potentials. The capacitors

on VCC and VPLUS must be large enough to keep the circuit

operating correctly during the 15ms short-circuit event.

The charger will stop within a few microseconds, leaving

a small current which must be provided by the capacitor

on VPLUS. The recommended minimum values (1μF on

VPLUS and 2μF on VCC, including tolerances) should keep

the LTC1760 operating above the UVLO trip voltage long

enough to perform the short-circuit function when the

input voltages are greater than 8V. Increasing the capaci-

tor across VCC to 4.7μF will allow operation down to the

recommended 6V minimum.

8.3 Emergency Turn-Off

All of the PowerPath switches can be forced off by set-

ting the DCDIV pin to a voltage between 8V and 10V. This

will have the same effect as a short-circuit event. DCDIV

must be less than 5V and VPLUS must decrease below

the UVLO threshold to re-enable the PowerPath switches.

The LTC1760 can recover from this condition without

removing power. Contact Applications Engineering for

more information.

8.4 Power-Up Strategy

All three PowerPath switches are turned on after VPLUS

exceeds the UVLO threshold for more than 250ms. This

delay is to prevent oscillation from a turn-on transient

near the UVLO threshold.

9 The Voltage DAC Block

The voltage DAC (VDAC) is a delta-sigma modulator

which controls the effective value of an internal resistor,

RVSET = 7.2k, used to program the maximum charger volt-

age. Figure 7 is a simplified diagram of the VDAC operation.

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