LTC3770EUH#TR Linear Technology, LTC3770EUH#TR Datasheet - Page 17

• Manufacturer Part Number: LTC3770EUH#TR
• Manufacturer: Linear Technology

Specifications of LTC3770EUH#TR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
tracking mode, V
implement the ratiometric tracking, the ratio of the divider
should be exactly the same as the master IC’s feedback
divider. Note that the pin Z1 of the slave IC should be tied
to INTV
in both tracking modes or it will introduce a small error
on the tracking voltage depending on the absolute values
of the tracking resistive divider.
By selecting different resistors, the LTC3770 can achieve
different modes of tracking including the two in Figure 7.
So which mode should be programmed? While either
mode in Figure 7 satisfi es most practical applications,
Figure 9. Equivalent Input Circuit of Error Amplifi er
TRACK/SS2
CC
so that the internal soft-start current is disabled
V
0.6V
FB2
D1
TRACK/SS2
OUT1
V
OUT1
PIN
TO
(7a) Coincident Tracking
must be set higher than V
I
D2
(8a) Coincident Tracking Setup
R3
R4
I
D3
TIME
+
–
R1
R2
Figure 7. Two Different Modes of Output Voltage Tracking
Figure 8. Setup for Coincident and Ratiometric Tracking
EA2
TO
V
PIN
FB1
V
PIN
FB2
TO
3770 F09
V
V
OUT1
OUT2
OUT2
R3
R4
. To
V
OUT2
there do exist some tradeoffs. The ratiometric mode saves
a pair of resistors, but the coincident mode offers better
output regulation. This can be better understood with the
help of Figure 9. At the input stage of the slave IC’s error
amplifi er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
is used to match the shifted common mode voltage. The
top two current sources are of the same amplitude. In the
coincident mode, the TRACK/SS voltage is substantially
higher than 0.6V at steady state and effectively turns off
D1. D2 and D3 will therefore conduct the same current
and offer tight matching between V
precision 0.6V reference. In the ratiometric mode, however,
TRACK/SS equals 0.6V at steady state. D1 will divert part
of the bias current to make V
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
IC’s output experiences dynamic excursion (under load
transient, for example), the slave IC output will be affected
as well. For better output regulation, use the coincident
tracking mode instead of ratiometric.
TRACK/SS2
V
OUT1
PIN
(8b) Ratiometric Tracking Setup
TO
(7b) Ratiometric Tracking
R1
R2
TO
V
PIN
FB1
TIME
V
PIN
FB2
TO
FB2
R3
R4
3770 F08
slightly lower than 0.6V.
3770 F07
V
V
V
FB2
OUT2
OUT1
OUT2
LTC3770
and the internal
17
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