LTC3812-5_15 Datasheet, PDF (19/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
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 significantly 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 amplifier. 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 flow 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 simplified 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,
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