LTC3577 datasheet(21/52 Pages) LINER

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LTC3577/LTC3577-1

3577f

OPERATION

Ideal Diode from BAT to VOUT

The LTC3577 has an internal ideal diode as well as a con-

troller for an optional external ideal diode. Both the internal

and the external ideal diodes respond quickly whenever

VOUT drops below BAT. If the load increases beyond the

input current limit, additional current will be pulled from

the battery via the ideal diodes. Furthermore, if power to

VBUS (USB) or VOUT (external wall power or high voltage

regulator) is removed, then all of the application power

will be provided by the battery via the ideal diodes. The

ideal diodes are fast enough to keep VOUT from dropping

signiﬁcantly with just the recommended output capacitor

(see Figure 2). The ideal diode consists of a precision ampli-

ﬁer that enables an on-chip P-channel MOSFET whenever

the voltage at VOUT is approximately 15mV (VFWD) below

the voltage at BAT. The resistance of the internal ideal

diode is approximately 200mΩ. If this is sufﬁcient for

the application, then no external components are neces-

sary. However, if lower resistance is needed, an external

P-channel MOSFET can be added from BAT to VOUT. The

IDGATE pin of the LTC3577 drives the gate of the external

P-channel MOSFET for automatic ideal diode control. The

source of the MOSFET should be connected to VOUT and

the drain should be connected to BAT. Capable of driving a

1nF load, the IDGATE pin can control an external P-channel

MOSFET having extremely low on-resistance.

Using the WALL Pin to Detect the Presence of an

External Power Source

The WALL input pin can be used to identify the presence

of an external power source (particularly one that is not

subject to a ﬁxed current limit like the USB VBUS input).

Typically, such a power supply would be a 5V wall adapter

output or the low voltage output of a high voltage buck

regulator (speciﬁcally, LT3480, LT3653 or LT3505). When

the wall adapter output (or buck regulator output) is con-

nected directly to the WALL pin, and the voltage exceeds

the WALL pin threshold, the USB power path (from VBUS

to VOUT) will be disconnected. Furthermore, the ACPR pin

will be pulled low. In order for the presence of an external

power supply to be acknowledged, both of the following

conditions must be satisﬁed:

1. The WALL pin voltage must exceed approximately

4.3V.

2. The WALL pin voltage must be greater than 75mV

above the BAT pin voltage.

The input power path (between VBUS and VOUT) is re-

enabled and the ACPR pin is pulled high when either of

the following conditions is met:

1. The WALL pin voltage falls to within 25mV of the BAT

pin voltage.

2. The WALL pin voltage falls below 3.2V.

Each of these thresholds is suitably ﬁltered in time to

prevent transient glitches on the WALL pin from falsely

triggering an event.

External HV Buck Control Through the VC Pin

The WALL, ACPR and VC pins can be used in conjunction

with an external high voltage buck regulator such as the

LT3480, LT3505 or LT3653 to provide power directly to the

VOUT pin as shown in Figures 3 to 5 (Consult factory for

complete list of approved high voltage buck regulators).

When the WALL pin voltage exceeds 4.3V, VC pin control

circuitry is enabled and drives the VC pin of the LT3480,

LT3505 or LT3653. The VC pin control circuitry is designed

so that no compensation components are required on the

VC node. The voltage at the VOUT pin is regulated to the

larger of (BAT + 300mV) or 3.6V as shown in Figure 6.

4.0V

3.8V

VOUT

3.6V

500mA

–500mA

IBAT

IVOUT

LOAD

10μs/DIV

CHARGE

DISCHARGE

3577 F02

VBAT = 3.8V

VBUS = 5V

5x MODE

COUT = 10μF

Figure 2. Ideal Diode Transient Response

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