Datasheet

LTC3677-3
30
36773f
operaTion
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 volt-
age of 3.3V). When this happens, the P-Channel MOSFET
switch duty cycle increases until it is turned on continu-
ously 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 patented
circuitry to limit the slew rate of the switch node (SW1,
SW2 and SW3). This new circuitry is designed to transi-
tion the switch node over a period of a few nanoseconds,
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 adjust-
able via the I
2
C registers SLEWCTL1 and SLEWCTL2. 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) setting. 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.
Figure 9. V
OUT3
(1.2V) Efficiency and Power Loss vs I
OUT3
Figure 10. V
OUT3
(2.5V) Efficiency and Power Loss vs I
OUT3
I
OUT3
(µA)
0.01
EFFICIENCY (%)
POWER LOSS (mW)
60
80
100
100
36773 F09
40
20
50
70
90
30
10
0
10
100
1000
1
0.1
0.001
1 100.1 1000
Burst Mode
OPERATION
V
IN
= 3.8V
SW[1:0] =
00
01
10
11
I
OUT3
(µA)
0.01 0.1 10 1000
EFFICIENCY (%)
POWER LOSS (mW)
60
80
100
100
36773 F10
40
20
50
70
90
30
10
0
10
100
1000
1
0.1
0.01
1
Burst Mode
OPERATION
V
IN
= 3.8V
SW[1:0] =
00
01
10
11