Datasheet

LTC3852
19
3852f
APPLICATIONS INFORMATION
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.
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
(5a) Coincident Tracking Setup
R3
V
OUT
R4
TO
V
FB
PIN
R3
V
MASTER
R4
TO
TRACK/SS
PIN
3852 F05a
(5b) Ratiometric Tracking Setup
R1 R3
V
OUT
R4R2
3852 F05b
TO
V
FB
PIN
TO
TRACK/SS
PIN
V
MASTER
Figure 5. Setup for Coincident and Ratiometric Tracking
–
+
II
D1
TRACK/SS
0.8V
V
FB
D2
D3
3852 F06
EA
Figure 6. Equivalent Input Circuit of Error Amplifi er
and the BOOST pin follows. With the topside MOSFET on,
the boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
The value of the boost capacitor C
B
needs to be 100 times
that of the total input capa citance of the topside MOSFET.
The reverse break down of the external Schottky diode
must be greater than V
IN(MAX)
.
Undervoltage Lockout
The LTC3852 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
CC
voltage
to ensure that an adequate gate-drive voltage is present. It
locks out switching action when INTV
CC
falls below 3.25V.
To prevent oscillation when there is a disturbance on the
INTV
CC
, the UVLO comparator has 400mV of preci sion
hysteresis.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pin has a precision
turn-on reference of 1.25V, one can use a resistor divider
to V
IN
to turn on the IC when V
IN
is high enough.
C
IN
Selection
In continuous mode, the source current of the top N-channel
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
I
RMS
≅ I
O(MAX)
V
OUT
V
IN
V
IN
V
OUT
–1
⎛
⎝
⎜
⎞
⎠
⎟
1/ 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
O(MAX)
/2. This simple worst-case condition is com monly
used for design because even signifi cant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design. Always consult
the manufacturer if there is any question.