LTC1624
U
W U U
APPLICATIONS INFORMATION
monlyusedfordesignbecauseevensignificantdeviations
donotoffermuchrelief.Notethatcapacitormanufacturer’s
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
temperaturethanrequired.Severalcapacitorsmayalsobe
paralleled to meet size or height requirements in the
design. Always consult the manufacturer if there is any
question.
ratings that are ideal for input capacitor applications.
Consult the manufacturer for other specific recommend-
ations.
INTVCC Regulator
An internal regulator produces the 5V supply that powers
the drivers and internal circuitry within the LTC1624.
Good VIN bypassing is necessary to supply the high
transient currents required by the MOSFET gate drivers.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple (∆VOUT) is determined by:
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1624 to be
exceeded. The supply current is dominated by the gate
charge supply current as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics table. For example, the LTC1624
is limited to less than 17mA from a 30V supply:
1
∆V
≈ ∆I ESR +
L
OUT
4fC
OUT
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆IL increases
with input voltage. With ∆IL = 0.4IOUT(MAX) the output
ripplewillbelessthan100mVatmaximumVIN,assuming:
TJ = 70°C + (17mA)(30V)(110°C/W) = 126°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum VIN.
COUT Required ESR < 2RSENSE
Manufacturers such as Nichicon, United Chemicon and
SANYO should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
dielectric capacitor available from SANYO has the lowest
ESR(size)productofanyaluminumelectrolyticatasome-
what higher price. Once the ESR requirement for COUT has
been met, the RMS current rating generally far exceeds
the IRIPPLE(P-P) requirement.
Step-Down Converter: Topside MOSFET Driver
Supply (CB, DB)
AnexternalbootstrapcapacitorCB connectedtotheBOOST
pinsuppliesthegatedrivevoltageforthetopsideMOSFET.
Capacitor CB in the functional diagram is charged through
internal diode DB from INTVCC when the SW pin is low.
When the topside MOSFET is to be turned on, the driver
places the CB voltage across the gate to source of the
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage SW rises to VIN
and the BOOST pin rises to VIN + INTVCC. The value of the
boost capacitor CB needs to be 50 times greater than the
total input capacitance of the topside MOSFET. In most
applications 0.1µF is adequate.
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalums, available in case
heightsrangingfrom2mmto4mm. Othercapacitortypes
include SANYO OS-CON, Nichicon WF series and Sprague
595Dseriesandthenewceramics.Ceramiccapacitorsare
nowavailableinextremelylowESRandhighripplecurrent
Significant efficiency gains can be realized by supplying
topsidedriveroperatingvoltage fromtheoutput,sincethe
VIN current resulting from the driver and control currents
will be scaled by a factor of (Duty Cycle)/(Efficiency). For
5V regulators this simply means connecting the BOOST
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Linear [ Linear ]
