LTC1435
U
W U U
APPLICATIONS INFORMATION
where δ is the temperature dependency of RDS(ON) and k
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.
is a constant inversely related to the gate drive current.
Both MOSFETs have I2R losses while the topside
N-channel equation includes an additional term for tran-
sition losses, which are highest at high input voltages.
For VIN < 20V the high current efficiency generally im-
proves with larger MOSFETs, while for VIN > 20V the
transition losses rapidly increase to the point that the use
of a higher RDS(ON) device with lower CRSS actual pro-
videshigherefficiency.ThesynchronousMOSFETlosses
are greatest at high input voltage or during a short circuit
when the duty cycle in this switch is nearly 100%. Refer
to the Foldback Current Limiting section for further
applications information.
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 approximated by:
1
∆V
≈ ∆I ESR +
L
OUT
4fC
OUT
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltageMOSFETs.CRSSisusuallyspecifiedintheMOSFET
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
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
ripple will be less than 100mV at max VIN assuming:
COUT required ESR < 2RSENSE
The Schottky diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two large
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on and storing charge
during the dead-time, which could cost as much as 1% in
efficiency. A 1A Schottky is generally a good size for 3A
regulators.
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR(size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.
CIN and COUT Selection
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
surfacemountconfigurations. Inthecaseoftantalum, itis
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalum, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON, Nichicon PL series and Sprague
593Dand 595Dseries.Consultthemanufacturerforother
specific recommendations.
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle VOUT
/
VIN. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
1/2
]
V
V − V
OUT
(
)
OUT IN
[
C required I
≈I
IN
RMS MAX
V
IN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monlyusedfordesignbecauseevensignificantdeviations
10
Linear [ Linear ]
