LTC3608(RevC) Просмотр технического описания (PDF) - Linear Technology
Номер в каталоге
Компоненты Описание
производитель
LTC3608 Datasheet PDF : 26 Pages
| |||
LTC3608
Applications Information
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores.
A variety of inductors designed for high current, low volt-
age applications are available from manufacturers such as
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
CIN and COUT Selection
The input capacitance, CIN, is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
IRMS
≅ IOUT(MAX)
VOUT
VIN
VIN – 1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/2. This simple worst-case condition is
commonly used for design because even significant de-
viations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
the capacitor.
The selection of COUT is primarily determined by the
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔVOUT is approximately
bounded by:
ΔVOUT
≤
ΔIL
⎛
⎝⎜
ESR
+
1
8fCOUT
⎞
⎠⎟
Since ΔIL increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
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 providing that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteris-
tics but can have a high voltage coefficient and audible
piezoelectric 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 ringing from inrush currents and switching does not
pose an overvoltage hazard to the power switches and
controller. To dampen input voltage transients, add a small
5µF to 50µF aluminum electrolytic capacitor with an ESR in
the range of 0.5Ω to 2Ω. 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.
Top MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor, CB, connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode DB from INTVCC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to VIN and the BOOST pin rises
to approximately VIN + INTVCC. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications an 0.1µF to 0.47µF, X5R
or X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when in-
ductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
with changes in VIN. Tying the FCB pin below the 0.6V
3608fc
14
Share Link: 