LTC1436AIGN View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1436AIGN' PDF : 28 Pages View PDF
LTC1436A
LTC1436-PLL-A/LTC1437A
APPLICATIONS INFORMATION
where δ is the temperature dependency of RDS(ON) and k
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 transi-
tion losses, which are highest at high input voltages. For
VIN < 20V the high current efficiency generally improves
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 provides higher
efficiency. The synchronous MOSFET losses 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 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
voltage MOSFETs. CRSS is usually specified in the MOSFET
characteristics. The constant k = 2.5 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
The Schottky diode D1 shown in Figure 1 serves two
purposes. During continuous synchronous operation, D1
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. During low current operation, D1 oper-
ates in conjunction with the small top MOSFET to provide
an efficient low current output stage. A 1A Schottky is
generally a good compromise for both regions of opera-
tion due to the relatively small average current.
CIN and COUT Selection
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
VOUT VIN − VOUT
CIN Required IRMS ≈ IMAX
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’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
temperature than required. Several capacitors may also be
paralleled to meet size or height requirements in the
design. Always consult the manufacturer if there is any
question.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisified, the capacitance is adequate for filtering.
The output ripple (∆VOUT) is approximated by:
∆VOUT
≈
∆ILESR
+
1
4fCOUT
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 maximum VIN, assuming:
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)
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.
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
heights ranging from 2mm to 4mm. Other capacitor types
14
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