LTC3770_15 Datasheet, PDF (17/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

APPLICATIONS INFORMATION
V_OUT1
V_OUT2
LTC3770
V_OUT1
V_OUT2
TIME
(7a) Coincident Tracking
TIME
3770 F07
(7b) Ratiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TRACK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(8a) Coincident Tracking Setup
V_OUT1
TO
TRACK/SS2
PIN
R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
3770 F08
(8b) Ratiometric Tracking Setup
Figure 8. Setup for Coincident and Ratiometric Tracking
I
I
D1
TRACK/SS2
0.6V
V_FB2
+
D2
EA2
–
D3
3770 F09
Figure 9. Equivalent Input Circuit of Error Amplifier
tracking mode, V_OUT1 must be set higher than V_OUT2. To
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_CC so 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,
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_FB2 and the internal
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_FB2 slightly lower than 0.6V.
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.
3770fc
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