Datasheet

LTC3853
19
3853fa
Figure 7. Equivalent Input Circuit of Error Amplifier
Figure 5. Two Different Modes of Output Voltage Tracking
Figure 6. Setup for Coincident and Ratiometric Tracking
APPLICATIONS INFORMATION
TIME
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3853 F05a
V
OUT1
V
OUT2
TIME
3853 F05b
(5b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(6a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3853 F06
(6b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
–
+
I I
D1
TK/SS2
0.8V
V
FB2
D2
D3
3853 F07
EA2
is used to match the shifted common mode voltage. The
top two current sources are of the same amplitude. In the
coincident mode, the TK/SS voltage is substantially higher
than 0.8V 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
0.8V reference. In the ratiometric mode, however, TK/SS
equals 0.8V at steady state. D1 will divert part of the bias
current to make V
FB2
slightly lower than 0.8V.
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
channel’s output experiences dynamic excursion (under
load transient, for example), the slave channel output will
be affected as well. For better output regulation, use the
coincident tracking mode instead of ratiometric.
INTV
CC
Regulators and EXTV
CC
The LTC3853 features an NPN linear regulator that supplies
power to INTV
CC
from the V
IN
supply. INTV
CC
powers the
gate drivers and much of the LTC3853’s internal circuitry.
The linear regulator regulates the voltage at the INTV
CC
pin
to 5V when V
IN
is greater than 6.5V. EXTV
CC
connects to
INTV
CC
through a P-channel MOSFET and can supply the
needed power when its voltage is higher than 4.7V. Each
of these can supply a peak current of 150mA and must
be bypassed to ground with a minimum of 1µF ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capacitor is used, an additional 0.1µF