LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 21

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
connected to a soft-start capacitor. All other LTC3708s
should have their RUN/SS pins pulled up to V
resistor between 50k and 300k. Figure 12 shows the circuit
with four outputs. Three of them are programmed in the
coincident mode while the fourth one tracks ratiometrically.
If output tracking is not needed, connect the TRACK pins
to V
CC
Figure 11. Alternative Setup for Coincident Tracking
. Do Not Float These Pins.
⎛
⎝ ⎝ ⎜
R
TO TRACK2 PIN
1
R
+
TO V
3
R
V
V
2
FB1
OUT1
OUT3
V
=
OUT1
PIN
V
OUT
0 6
.
R4
R2
1
– ,
1
R4
R2
R1
R2
R3
R
TO V
3708 F11
(12a) Circuit Setup
R5
R2
2
R
TO V
+
CC
1
100k
Figure 12. Four Outputs with Tracking and Ratiometric Sequencing
⎛
⎝ ⎜
R
R1
R2
CC
R
R
3
2
1
=
R4
R5
=
R
R
V
4
5
C
OUT
0 6
TRACK1
V
RUN/SS
SS
TRACK1
V
RUN/SS
=
FB1
FB1
V
.
TO V
OUT2
“MASTER”
V
LTC3708
LTC3708
“SLAVE”
1
OUT
0 6
−
FB2
.
TRACK2
TRACK2
1
,
2
PIN
R
R
V
V
−
3
2
FB2
FB2
CC
1
=
⎞
⎠ ⎟
V
with a
OUT
0 6
V
V
.
OUT2
OUT4
2
R3
R2
R5
R2
−
1
,
R
R
4
2
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the effi ciency and which change would
produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources account for most of the losses
in LTC3708 circuits:
1. DC I
MOSFETs, inductor and PC board traces and cause the
effi ciency to drop at high output currents. In continuous
mode, the average output current fl ows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
loss. For example, if R
the loss will range from 15mW up to 1.5W as the output
current varies from 1A to 10A.
=
V
OUT
0
. .
6
2
3
R Losses. These arise from the resistances of the
−
1
,
R
R
5
2
(12b) Output Voltage
=
V
OUT
0 6
.
4
TIME
DS(ON)
−
1
⎞
⎠ ⎟
= 0.01Ω and R
3708 F12
V
V
V
V
OUT1
OUT3
OUT4
OUT2
LTC3708
DS(ON)
L
= 0.005Ω,
, then the
21
3708fb
2
R