LTC3865-1_15 Datasheet, PDF (20/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
APPLICATIONS INFORMATION
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(5a) Coincident Tracking
TIME
3865 F05
(5b) Ratiometric Tracking
Figure 5. Two Different Modes of Output Voltage Tracking
V_OUT1
nR3
TO
TK/SS2
PIN
nR4
R1
TO TO
EA1 EA2
R2
V_OUT2
R3
R4
V_OUT1
nR1
TO
TK/SS2
PIN
nR2
R1
TO TO
EA1 EA2
R2
V_OUT2
R3
R4
38551 F06
(6a) Coincident Tracking Setup
(6b) Ratiometric Tracking Setup
Figure 6. Setup for Coincident and Ratiometric Tracking
TK/SS2
0.6V
V_FB2
I
I
D1 D2
+
EA2
–
D3
3865 F07
Figure 7. Equivalent Input Circuit of Error Amplifier
shown in Figure 6a. In this tracking mode, V_OUT1 must
be set higher than V_OUT2. To implement the ratiometric
tracking, the ratio of the V_OUT2 divider should be exactly
the same as the master channel’s internal feedback divider.
By selecting different resistors, the LTC3865/LTC3865-1
can achieve different modes of tracking including the two
in Figure 5.
So which mode should be programmed? The coincident
mode offers better output regulation. This can be better
understood with the help of Figure 7. At the input stage
of the slave channel’s error amplifier, two common an-
ode diodes are used to clamp the equivalent reference
20
voltage and an additional diode is used to match the shifted
common mode voltage. The top two current sources are
of the same magnitude. In coincident mode, the TK/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 at steady state. In the
ratiometric mode, however, TK/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 characteristics of the diode, it does
impose a finite amount of output voltage deviation.
3865fb