LTC3566-2_15 Datasheet, PDF(18/28 Page) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

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LTC3566-2_15 Datasheet, PDF (18/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

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LTC3566/LTC3566-2

OPERATION

Buck-Boost Regulator Burst Mode Operation

In Burst Mode operation, the buck-boost regulator uses

a hysteretic FB1 voltage algorithm to control the output

voltage. By limiting FET switching and using a hysteretic

control loop, switching losses are greatly reduced. In this

mode output current is limited to 50mA typical. While

operating in Burst Mode operation, the output capacitor

is charged to a voltage slightly higher than the regulation

point. The buck-boost converter then goes into a sleep

state, during which the output capacitor provides the

load current. The output capacitor is charged by charg-

ing the inductor until the input current reaches 275mA

typical and then discharging the inductor until the reverse

current reaches 0mA typical. This process is repeated

until the feedback voltage has charged to 6mV above the

regulation point. In the sleep state, most of the regulatorâs

circuitry is powered down, helping to conserve battery

power. When the feedback voltage drops 6mV below the

regulation point, the switching regulator circuitry is pow-

ered on and another burst cycle begins. The duration for

which the regulator sleeps depends on the load current

and output capacitor value. The sleep time decreases as

the load current increases. The maximum load current in

Burst Mode operation is 50mA. The buck-boost regulator

will not go to sleep if the current is greater than 50mA

and if the load current increases beyond this point while

in Burst Mode operation the output will lose regulation.

Burst Mode operation provides a signiï¬cant improve-

ment in efï¬ciency at light loads at the expense of higher

output ripple when compared to PWM mode. For many

noise-sensitive systems, Burst Mode operation might

be undesirable at certain times (i.e. during a transmit or

receive cycle of a wireless device), but highly desirable

at others (i.e. when the device is in low power standby

mode). The MODE pin is used to enable or disable Burst

Mode operation at any time, offering both low noise and

low power operation when they are needed.

Buck-Boost Regulator Soft-Start Operation

Soft-start is accomplished by gradually increasing the

reference voltage input to the error ampliï¬er over a 0.5ms

(typical) period. This limits transient inrush currents during

start-up because the output voltage is always in regulation.

Ramping the reference voltage input also limits the rate of

increase in the VC1 voltage which helps minimize output

overshoot during start-up. A soft-start cycle occurs when-

ever the buck-boost is enabled, or after a fault condition

has occurred (thermal shutdown or UVLO). A soft-start

cycle is not triggered by changing operating modes. This

allows seamless operation when transitioning between

Burst Mode operation and PWM mode.

Low Supply Operation

The LTC3566 family incorporates an undervoltage lockout

circuit on VOUT (connected to VIN1) which shuts down the

buck-boost regulator when VOUT drops below 2.6V. This

UVLO prevents unstable operation.

Table 2. USB Current Limit Settings

ILIM1

ILIM0

USB SETTING

1x Mode (USB 100mA Limit)

10x Mode (Wall 1A Limit)

Suspend

5x Mode (USB 500mA Limit)

Table 3. Switching Regulator Modes

MODE

SWITCHING REGULATOR MODE

PWM Mode

Burst Mode Operation

3566fb

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