LTC3577_15 Datasheet, PDF (33/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
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 SLEWCTL2.
This allows the user to optimize efficiency or EMI as neces-
sary with four different slew rate settings. The power-up
default is the fastest slew rate (highest efficiency) setting.
Figures 14 and 15 show the efficiency and power loss
graph for Buck3 programmed for 1.2V and 2.5V outputs.
Note that the power loss curves remain fairly constant for
both graphs yet changing the slew rate has a larger effect
on the 1.2V output efficiency. This is mainly because for
a given output current the 2.5V output is delivering more
than 2x the power than the 1.2V output. Efficiency will
always decrease and show more variation to slew rate as
the programmed output voltage is decreased.
Low Supply Operation
An undervoltage lockout circuit on V_OUT (V_OUT UVLO)
shuts down the step-down switching regulators when V_OUT
drops below about 2.7V. It is recommended that the step-
down switching regulator input supplies (V_IN12, V_IN3) be
connected to the power path output (V_OUT) directly. This
UVLO prevents the step-down switching regulators from
operating at low supply voltages where loss of regula-
tion or other undesirable operation may occur. If driving
the step-down switching regulator input supplies from
a voltage other than the V_OUT pin, the regulators should
not be operated outside the specified operating range as
operation is not guaranteed beyond this range.
Inductor Selection
Many different sizes and shapes of inductors are available
from numerous manufacturers. Choosing the right inductor
from such a large selection of devices can be overwhelming,
but following a few basic guidelines will make the selection
100
90
80
70
60
50
40
30
20
10
0
1.00E-05
1.00E+00
1.00e-01
1.00E-02
1.00E-0.3
I_OUT3 (mA)
Burst Mode
OPERATION
V_IN = 3.8V
SW[1:0] =
00
01
10
11
1.00E-01
1.00E-03
1.00E-04
1.00E-05
3577 F14
Figure 14. V_OUT3 (1.2V) Efficiency and Power Loss vs I_OUT3
100
90
80
70
60
50
40
30
20
10
0
1.00E-05
1.00E+00
1.00e-01
1.00E-02
1.00E-0.3
I_OUT3 (mA)
Burst Mode
OPERATION
V_IN = 3.8V
SW[1:0] =
00
01
10
11
1.00E-01
1.00E-03
1.00E-04
1.00E-05
3577 F15
Figure 15. V_OUT3 (2.5V) Efficiency and Power Loss vs I_OUT3
3577fa
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