LTC3576_15 datasheet(21/48 Pages) LINER

21 / 48 page

LTC3576/LTC3576-1

3576fb

OPERATION

high voltage switching regulator. As a step-up converter,

the bidirectional switching regulator produces 5V on

VBUS and is capable of delivering at least 500mA. USB

on-the-go can be enabled by either the external control

pin, ENOTG, or via I2C. Figure 3 shows the power ﬂow

in step-up mode.

An undervoltage lockout circuit monitors VOUT and pre-

vents step-up conversion until VOUT rises above 2.8V. To

prevent backdriving of VBUS when input power is available,

the VBUS undervoltage lockout circuit prevents step-up

conversion if VBUS is greater than 4.3V at the time step-up

mode is enabled. The switching regulator is also designed

to allow true output disconnect by eliminating body diode

conduction of the internal PMOS switch. This allows VBUS

to go to zero volts during a short-circuit condition or while

shut down, drawing zero current from VOUT.

The voltage regulation loop is compensated by the capaci-

tance on VBUS. A 4.7μF MLCC is required for loop stability.

Additional capacitance beyond this value will improve

transient response. The VBUS voltage has approximately

3% load regulation up to an output current of 500mA. At

light loads, the switching regulator goes into Burst Mode

operation. The regulator will deliver power to VBUS until it

reaches 5.1V after which the NMOS and PMOS switches

shut off. The regulator delivers power again to VBUS once

it falls below 5.1V.

The switching regulator features both peak inductor and

average output current limit. The peak current mode

architecture limits peak inductor current on a cycle-by-

cycle basis. The peak current limit is equal to VBUS/2Ω to

a maximum of 1.8A so that in the event of a sudden short

circuit, the current limit will fold back to a lower value.

In step-up mode, the voltage on CLPROG represents the

average output current of the switching regulator when

a programming resistor and an averaging capacitor are

connected from CLPROG to GND. With a 3.01k resistor

on CLPROG, the bidirectional switching regulator has an

output current limit of 680mA. As the output current ap-

–

0.3V

1.15V

3.6V

CLPROG

ISWITCH/N

+ –

15mV

OmV

IDEAL

DIODE

PWM AND

GATE DRIVE

AVERAGE VBUS OUTPUT

CURRENT LIMIT

CONTROLLER

VBUS

VOLTAGE

CONTROLLER

VOUT VOLTAGE

CONTROLLER

CONSTANT CURRENT

CONSTANT VOLTAGE

BATTERY CHARGER

IDGATE

VOUT

3.5V TO

(BAT + 0.3V)

TO SYSTEM LOAD

OPTIONAL EXTERNAL

IDEAL DIODE PMOS

SINGLE CELL

Li-Ion

3576 F03

BAT

TO USB

CABLE

OVGATE

VBUS

OVSENS

TO AUTOMOTIVE,

FIREWIRE, ETC.

ACPR

BAT + 0.3V

3.6V

VOUT

4.3V

WALL

Bat-Track HV CONTROL

VIN

HIGH VOLTAGE

STEP-DOWN

SWITCHING

REGULATOR

OVERVOLTAGE PROTECTION

+ –

BATTERY POWER

HV INPUT

Figure 3. PowerPath Block Diagram—USB On-the-Go

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