LTC1538-AUX/LTC1539
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are given by:
generally a good compromise for both regions of opera-
tion due to the relatively small average current.
PMAIN
=
VOUT
VIN
(IMAX)2(1+
)δ RDS(ON)
+
k(VIN)1.85 (IMAX)(CRSS)(f)
( ) ( ) PSYNC
=
VIN
– VOUT
VIN
IMAX 2
1+ δ
RDS(ON)
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
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:
[ ] ( ) CIN Required IRMS ≈ IMAX
VOUT
VIN – VOUT
VIN
1/ 2
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT/2. This simple worst-case condition is commonly 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 satisfied the capacitance is adequate for filtering.
The output ripple (∆VOUT) is approximated by:
∆VOUT
≈
∆IL
ESR
+
1
4 fC 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
ripple will be less than 100mV at max 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
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