ltc3770 Linear Technology Corporation, ltc3770 Datasheet - Page 17

• Manufacturer Part Number: ltc3770
• Description: Fast No Rsense Step-down Synchronous Controller With Margining, Tracking And Pll
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
mode, V
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
that the internal soft-start current is disabled 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 satisfies most practical applications,
Figure 9. Equivalent Input Circuit of Error Amplifier
TRACK/SS2
OUT1
0.6V
V
FB2
must be set higher than V
D1
TRACK/SS2
V
U
OUT1
PIN
TO
(7a) Coincident Tracking
I
(8a) Coincident Tracking Setup
D2
U
R3
R4
I
D3
TIME
+
–
Figure 7. Two Different Modes of Output Voltage Tracking
R1
R2
Figure 8. Setup for Coincident and Ratiometric Tracking
W
EA2
TO
V
PIN
FB1
OUT2
. To implement
V
PIN
FB2
TO
3770 F09
U
V
V
OUT1
OUT2
R3
R4
CC
so
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
amplifier, 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
sion 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 finite 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
FB2
R3
R4
slightly lower than 0.6V.
3770 F08
3770 F07
and the internal preci-
V
OUT2
V
V
OUT1
OUT2
LTC3770
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