Datasheet

LTC3851-1
16
38511fa
APPLICATIONS INFORMATION
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
FB
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
FB
slightly lower than 0.8V.
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
supply’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
Regulator
The LTC3851-1 features a PMOS low dropout linear
regula tor (LDO) that supplies power to INTV
CC
from the
V
IN
supply. INTV
CC
powers the gate drivers and much of
the LTC3851-1 ’s internal circuitry. The LDO regulates the
voltage at the INTV
CC
pin to 5V.
The LDO can supply a peak current of 50mA and must
be bypassed to ground with a minimum of 2.2F ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capaci tor is used, an additional 0.1F
ceramic capacitor placed directly adjacent to the INTV
CC
and GND pins is highly recommended. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3851-1 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, is supplied by the 5V LDO.
Power dissipation for the IC in this case is highest and
is approximately equal to V
IN
• I
INTVCC
. The gate charge
current is dependent on operating frequency as discussed
in the Effi ciency Considerations section. The junction tem-
perature can be estimated by using the equa tions given in
Note 3 of the Electrical Characteristics. For example, the
LTC3851-1 INTV
CC
current is limited to less than 17mA
from a 36V supply in the GN package:
T
J
= 70°C + (17mA)(36V)(90°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= INTV
CC
) at maximum V
IN
.
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin
Figure 4. Setup for Coincident and Ratiometric Tracking
Figure 5. Equivalent Input Circuit of Error Amplifi er
R3
V
OUT
R4
(4a) Coincident Tracking Setup
TO
V
FB
PIN
R3
V
MASTER
R4
TO
TK/SS
PIN
R1 R3
V
OUT
R4R2
38511 F04
(4b) Ratiometric Tracking Setup
TO
V
FB
PIN
TO
TK/SS
PIN
V
MASTER
–
+
II
D1
TK/SS
0.8V
V
FB
D2
D3
38511 F05
EA