ltc3566 Linear Technology Corporation, ltc3566 Datasheet - Page 18

• Manufacturer Part Number: ltc3566
• Description: High Ef?ciency Usb Power Manager Plus 1a Buck-boost Converter
• Manufacturer: Linear Technology Corporation

LTC3566
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 charging 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 regulator’s circuitry
is powered down, helping to conserve battery power.
When the output voltage drops 6mV below the regulation
point, the switching regulator circuitry is powered 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 signifi cant improvement in ef-
fi 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.
18
Table 2. USB Current Limit Settings
Table 3. Switching Regulator Modes
Buck-Boost Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
reference voltage input to the error amplifi er over a 0.5ms
(typical) period. This limits transient inrush currents during
start-up because the output voltage is always “in regula-
tion”. Ramping the reference voltage input also limits the
rate of increase in the V
output overshoot during start-up. A soft-start cycle oc-
curs whenever 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 incorporates an undervoltage lockout circuit
on V
boost regulator when V
prevents unstable operation.
OUT
ILIM1
0
0
1
1
(connected to V
MODE
0
1
ILIM0
OUT
0
1
0
1
C1
IN1
voltage which helps minimize
) which shuts down the buck-
drops below 2.6V. This UVLO
SWITCHING REGULATOR MODE
PWM Mode
Burst Mode Operation
USB SETTING
1x Mode (USB 100mA Limit)
10x Mode (Wall 1A Limit)
Suspend
5x Mode (USB 500mA Limit)
3566p