ltc3851-1 Linear Technology Corporation, ltc3851-1 Datasheet - Page 16

• Manufacturer Part Number: ltc3851-1
• Description: Synchronous Step-down Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LTC3851-1
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
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
The LTC3851-1 features a PMOS low dropout linear
regula tor (LDO) that supplies power to INTV
V
the LTC3851-1 ’s internal circuitry. The LDO regulates the
voltage at the INTV
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
16
IN
supply. INTV
CC
Figure 5. Equivalent Input Circuit of Error Amplifi er
Regulator
TK/SS
0.8V
V
FB
CC
D1
FB
CC
V
powers the gate drivers and much of
MASTER
slightly lower than 0.8V.
I
pin to 5V.
TK/SS
D2
(4a) Coincident Tracking Setup
PIN
TO
38511 F05
I
FB
D3
and the internal precision
+
–
R3
R4
EA
Figure 4. Setup for Coincident and Ratiometric Tracking
V
PIN
TO
FB
CC
R3
R4
from the
V
OUT
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
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
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
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
from a 36V supply in the GN package:
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
Topside MOSFET Driver Supply (C
An external bootstrap capacitor C
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
though external diode D
V
MASTER
T
TK/SS
J
PIN
(4b) Ratiometric Tracking Setup
= 70°C + (17mA)(36V)(90°C/W) = 125°C
TO
CC
) at maximum V
R1
R2
CC
CC
current is limited to less than 17mA
B
in the Functional Diagram is charged
V
PIN
current, which is dominated by the
TO
FB
B
IN
from INTV
.
IN
R3
R4
38511 F04
• I
INTVCC
V
OUT
B
B
, D
CC
connected to the
. The gate charge
when the SW pin
B
)
38511f
CC