LTC3876_15 Datasheet, PDF (26/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount pack-
ages. 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. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications provided that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics
but can have a high voltage coefficient and audible piezo-
electric effects. The high-Q of ceramic capacitors with trace
inductance can also lead to significant ringing. When used
as input capacitors, care must be taken to ensure that ring-
ing from inrush currents and switching does not pose an
overvoltage hazard to the power switches and controller.
For high switching frequencies, reducing output ripple and
better EMI filtering may require small value capacitors that
have low ESL (and correspondingly higher self-resonant
frequencies) to be placed in parallel with larger value
capacitors that have higher ESL. This will ensure good
noise and EMI filtering in the entire frequency spectrum
of interest. Even though ceramic capacitors generally
have good high frequency performance, small ceramic
capacitors may still have to be parallel connected with
large ones to optimize performance.
High performance through-hole capacitors may also be
used, but an additional ceramic capacitor in parallel is
recommended to reduce the effect of their lead inductance.
Remember also to place high frequency decoupling capaci-
tors as close as possible to the power pins of the load.
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
DRV_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 approximately 100 times the gate charge
required by the top MOSFET. In most applications a 0.1μF
to 0.47μF, X5R or X7R dielectric capacitor is adequate. It
is recommended that the BOOST capacitor be no larger
than 10% of the DRV_CC capacitor, C_DRVCC, to ensure that
the C_DRVCC can supply the upper MOSFET gate charge
and BOOST capacitor under all operating conditions. Vari-
able frequency in response to load steps offers superior
transient performance but requires higher instantaneous
gate drive. Gate charge demands are greatest in high
frequency low duty factor applications under high load
steps and at start-up.
DRV_CC Regulator and EXTV_CC Power
The LTC3876 features a PMOS low dropout (LDO) linear
regulator that supplies power to DRV_CC from the V_IN supply.
The LDO regulates its output at the DRV_CC1 pin to 5.3V.
The LDO can supply a maximum current of 100mA and
must be bypassed to ground with a minimum of 4.7μF
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to minimize interaction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3876 to be
exceeded, especially if the LDO is active and provides
DRV_CC. Power dissipation for the IC in this case is high-
est and is approximately equal to V_IN • I_DRVCC. The gate
charge current is dependent on operating frequency as
discussed in the Efficiency Considerations section. The
junction temperature can be estimated by using the equa-
tion given in Note 2 of the Electrical Characteristics. For
example, when using the LDO, LTC3876’s DRV_CC current
is limited to less than 52mA from a 38V supply at T_A =
70°C in the FE package:
T_J = 70°C + (52mA)(38V)(28°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 at maximum V_IN.
3876f
26