LTC3853 Datasheet, PDF (19/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

APPLICATIONS INFORMATION
V_OUT1
V_OUT2
LTC3853
V_OUT1
V_OUT2
TIME
3853 F05a
(5a) Coincident Tracking
TIME
3853 F05b
(5b) Ratiometric Tracking
Figure 5. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(6a) Coincident Tracking Setup
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
3853 F06
(6b) Ratiometric Tracking Setup
Figure 6. Setup for Coincident and Ratiometric Tracking
D1
TK/SS2
0.8V
V_FB2
I
I
+
D2
EA2
–
D3
3853 F07
Figure 7. Equivalent Input Circuit of Error Amplifier
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 regualtor 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
3853f
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