Datasheet

LTC3851A
16
3851afa
applicaTions inForMaTion
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 finite 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 LTC3851A 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 LTC3851A ’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 2.2μF ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capaci tor is used, an additional 0.1μF
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 LTC3851A 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 Efficiency 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
LTC3851A INTV
CC
current is limited to less than 14mA
from a 36V supply in the GN package:
T
J
= 70°C + (14mA)(36V)(110°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
is low. When the topside MOSFET is to be turned on, the
driver places the C
B
voltage across the gate source of the
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage, SW, rises to V
IN
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
3851A F04
(4b) Ratiometric Tracking Setup
TO
V
FB
PIN
TO
TK/SS
PIN
V
MASTER
–
+
I I
D1
TK/SS
0.8V
V
FB
D2
D3
3851A F05
EA
Figure 4. Setup for Coincident and Ratiometric Tracking
Figure 5. Equivalent Input Circuit of Error Amplifier