LTC3577-3 Datasheet, PDF (32/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
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.
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
process much simpler. The step-down switching regula-
tors are designed to work with inductors in the range of
2.2μH to 10μH. For most applications a 4.7μH inductor is
suggested for step-down switching regulators providing
up to 500mA of output current while a 3.3μH inductor is
suggested for step-down switching regulators providing
up to 800mA. Larger value inductors reduce ripple current,
which improves output ripple voltage. Lower value induc-
tors result in higher ripple current and improved transient
response time, but will reduce the available output current.
To maximize efficiency, choose an inductor with a low DC
resistance. For a 1.2V output, efficiency is reduced about
2% for 100mΩ series resistance at 400mA load current,
and about 2% for 300mΩ series resistance at 100mA load
current. Choose an inductor with a DC current rating at
least 1.5 times larger than the maximum load current to
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
357734 F09
Figure 9. V_OUT3 (1.2V) Efficiency and Power Loss vs I_OUT3
32
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
357734 F10
Figure 10. V_OUT3 (2.5V) Efficiency and Power Loss vs I_OUT3
357734fa