LTC3812-5
19
38125fc
handling and load step requirements. Dry tantalum, special
polymer and aluminum electrolytic capacitors are available
in surface mount packages. Special polymer capacitors
offer very low ESR but have lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density but it is important to only use types
that have been surge tested for use in switching power
supplies. Several excellent surge-tested choices are the
AVX TPS and TPSV or the KEMET T510 series. Aluminum
electrolytic capacitors have signifi cantly higher ESR, but
can be used in cost-driven applications providing that
consideration is given to ripple current ratings and long
term reliability. Other capacitor types include Panasonic
SP and Sanyo POSCAPs.
OUTPUT VOLTAGE
The LTC3812-5 output voltage is set by a resistor divider
according to the following formula:
V
OUT
= 0.8V 1+
R
FB1
R
FB2
The external resistor divider is connected to the output as
shown in the Functional Diagram, allowing remote voltage
sensing. The resultant feedback signal is compared with
the internal precision 800mV voltage reference by the
error amplifi er. The internal reference has a guaranteed
tolerance of less than ±1%. Tolerance of the feedback
resistors will add additional error to the output voltage.
0.1% to 1% resistors are recommended.
TOP 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.
This capacitor is charged through diode D
B
from INTV
CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
IN
and the BOOST pin rises
to approximately V
IN
+ INTV
CC
. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1μF to 0.47μF, X5R
or X7R dielectric capacitor is adequate.
The reverse breakdown of the external diode, D
B
, must
be greater than V
IN(MAX)
. Another important consideration
for the external diode is the reverse recovery and reverse
leakage, either of which may cause excessive reverse
current to fl ow at full reverse voltage. If the reverse cur-
rent times reverse voltage exceeds the maximum allow-
able power dissipation, the diode may be damaged. For
best results, use an ultrafast recovery diode such as the
MMDL770T1.
IC/MOSFET DRIVER SUPPLY (INTV
CC
)
The LTC3812-5 drivers are supplied from the INTV
CC
and
BOOST pins (see Figure 3), which have an absolute maxi-
mum voltage of 14V. Since the main supply voltage, V
IN
is
typically much higher than 14V a separate supply for the IC
and driver power (INTV
CC
) must be used. The LTC3812-5
has integrated bias supply control circuitry that allows the
IC/driver supply to be easily generated from V
IN
and/or
V
OUT
with minimal external components. There are four
ways to do this as shown in the simplifi ed schematics of
Figure 4 and explained in the following sections.
Using the Linear Regulator for INTV
CC
Supply
In Mode 1, a small external SOT-23 MOSFET, controlled by
the NDRV pin, is used to generate a 5.5V start-up supply
from V
IN
. The small SOT-23 package can be used because
the NMOS is on continuously only during the brief start-up
period. As soon as the output voltage reaches 4.7V, the
LTC3812-5 turns off the external NMOS and the LTC3812-5
regulates the 5.5V supply from the EXTV
CC
pin (connected
to V
OUT
or a V
OUT
derived boost network) through an
internal low dropout regulator. For this mode to work
properly, EXTV
CC
must be in the range 4.7V < EXTV
CC
<
15V. If V
OUT
< 4.7V, a charge pump or extra winding can
be used to raise EXTV
CC
to the proper voltage, or alter-
natively, Mode 2 should be used as explained later in this
section. If V
OUT
is shorted or otherwise goes below the
minimum 4.5V threshold, the MOSFET connected to V
IN
is turned back on to maintain the 5.5V supply. However if
the output cannot be brought up within a timeout period,
APPLICATIONS INFORMATION
