LTC3577-4_15 Datasheet, PDF (31/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
better efficiency than pulse-skipping at light loads. The
step-down switching regulators allow mode transition
on-the-fly, providing seamless transition between modes
even under load. This allows the user to switch back and
forth between modes to reduce output ripple or increase
low current efficiency as needed. Burst Mode operation
is individually selectable for each step-down switching
regulator through the I²C register bits BK1BRST, BK2BRST
and BK3BRST.
Shutdown
The step-down switching regulators (Buck1, Buck2 and
Buck3) are shut down when the pushbutton circuitry is in
the power-down or power-off state. Step-down switching
regulator 3 (Buck3) can also be shut down by bringing the
EN3 input low. In shutdown all circuitry in the step-down
switching regulator is disconnected from the switching
regulator input supply leaving only a few nanoamps of
leakage current. The step-down switching regulator out-
puts are individually pulled to ground through internal 10k
resistors on the switch pin (SW1, SW2 or SW3) when in
shutdown.
Dropout Operation
It is possible for a step-down switching regulator’s input
voltage to approach its programmed output voltage (e.g., a
battery voltage of 3.4V with a programmed output voltage
of 3.3V). When this happens, the PMOS switch duty cycle
increases until it is turned on continuously at 100%. In this
dropout condition, the respective output voltage equals the
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Soft-Start Operation
Soft-start is accomplished by gradually increasing the peak
inductor current for each step-down switching regulator
over a 500μs period. This allows each output to rise slowly,
helping minimize inrush current required to charge up the
switching regulator output capacitor. A soft-start cycle
occurs whenever a given switching regulator is enabled.
A soft-start cycle is not triggered by changing operating
modes. This allows seamless output transition when
actively changing between operating modes.
Slew Rate Control
The step-down switching regulators contain new patent
pending circuitry to limit the slew rate of the switch node
(SW1, SW2 and SW3). This new circuitry is designed to
transition the switch node over a period of a few nanosec-
onds, significantly reducing radiated EMI and conducted
supply noise while maintaining high efficiency. Since
slowing the slew rate of the switch nodes causes efficiency
loss, the slew rate of the step-down switching regulators
is adjustable via the I²C registers SLEWCTL1 and SLEW-
CTL2. This allows the user to optimize efficiency or EMI as
necessary with four different slew rate settings. The power
up default is the fastest slew rate (highest efficiency) set-
ting. Figures 9 and 10 show the efficiency and power loss
graph for Buck3 programmed for 1.2V and 2.5V outputs.
100
1000
90
80
100
70
60
10
50
40
30
20
10
0
0.01 0.1
1
10
I_OUT3 (μA)
Burst Mode
OPERATION ₁
V_IN = 3.8V
SW[1:0] =
00
01 ^0.1
10
11
0.001
100 1000
357734 F09
Figure 9. V_OUT3 (1.2V) Efficiency and Power Loss vs I_OUT3
100
1000
90
80
100
70
60
10
50
40
30
20
10
0
0.01 0.1
1
10
I_OUT3 (μA)
Burst Mode
OPERATION ₁
V_IN = 3.8V
SW[1:0] =
00
01 ^0.1
10
11
0.01
100 1000
357734 F10
Figure 10. V_OUT3 (2.5V) Efficiency and Power Loss vs I_OUT3
357734fb
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