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LTC3708 Datasheet(PDF) 21 Page - Linear Technology

Part # LTC3708
Description  Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
PDF  32 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3708 Datasheet(HTML) 21 Page - Linear Technology

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LTC3708
21
3708fb
APPLICATIONS INFORMATION
connected to a soft-start capacitor. All other LTC3708s
should have their RUN/SS pins pulled up to VCC with a
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 VCC. Do Not Float These Pins.
Efficiency Considerations
The percent efficiency 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 efficiency 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 I2R Losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode, the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same RDS(ON), then the
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I2R
loss. For example, if RDS(ON) = 0.01Ω and RL = 0.005Ω,
the loss will range from 15mW up to 1.5W as the output
current varies from 1A to 10A.
R1
R4
VOUT1
VOUT2
TO TRACK2 PIN
TO VFB1 PIN
TO VFB2 PIN
R2
R5
R3
3708 F11
Figure 11. Alternative Setup for Coincident Tracking
RR
R
V
R
RR
R
R
V
OUT
OUT
12
30 6
1
1
23
4
50 6
1
12
+
=
+
==
−
⎛
.
–,
.
⎝⎝⎜
⎞
⎠⎟
TRACK1
VFB1
CSS
R3
R1
R2
R5
VOUT1
R2
R4
R2
R2
VFB2
TO VCC
TO VCC
VOUT2
R5
R2
VOUT4
R4
100k
R2
VOUT3
RUN/SS
TRACK2
LTC3708
“MASTER”
TRACK1
VFB1
VFB2
RUN/SS
TRACK2
LTC3708
“SLAVE”
(12a) Circuit Setup
TIME
3708 F12
(12b) Output Voltage
VOUT1
VOUT3
VOUT4
VOUT2
Figure 12. Four Outputs with Tracking and Ratiometric Sequencing
R
R
V
R
R
V
R
R
V
OUT
OUT
OUT
1
20 6
1
3
20 6
1
4
20
12
3
=−
=
−
=
.
,
.
,
..
,
.
6
1
5
20 6
1
4
−=
−
⎛
⎝⎜
⎞
⎠⎟
R
R
VOUT



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