LTC1735-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1735-1' PDF : 28 Pages View PDF
LTC1735-1
APPLICATIO S I FOR ATIO
MOSFET from turning on and storing charge during the
dead-time, which could cost as much as 1% in efficiency.
A 3A Schottky is generally a good size for 10A to 12A regu-
lators due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance. The diode may be omitted if the
efficiency loss can be tolerated.
CIN 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:
IRMS
≅
IO(MAX)
VOUT
VIN
VIN
VOUT
– 1 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.
COUT Selection
The selection of COUT is primarily determined by the
effective series resistance (ESR) to minimize voltage ripple.
The output ripple (∆VOUT) in continuous mode is deter-
mined by:
∆VOUT
≈
∆IL ESR
+
1
8fCOUT
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. Typically, once the ESR requirement
for COUT has been met, the RMS current rating generally
far exceeds the IRIPPLE(P-P) requirement. With ∆IL =
0.3IOUT(MAX) and allowing for 2/3 of the ripple due to ESR,
the output ripple will be less than 50mV at max VIN
assuming:
COUT required ESR < 2.2 RSENSE
COUT > 1/(8fRSENSE)
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran-
tees that the output voltage does not significantly dis-
charge during the operating frequency period due to ripple
current. The choice of using smaller output capacitance
increases the ripple voltage due to the discharging term
but can be compensated for by using capacitors of very
low ESR to maintain the ripple voltage at or below 50mV.
The ITH pin OPTI-LOOP compensation components can be
optimized to provide stable, high performance transient
response regardless of the output capacitors selected.
The selection of output capacitors for CPU or other appli-
cations with large load current transients is primarily de-
termined by the voltage tolerance specifications of the load.
The resistive component of the capacitor, ESR, multiplied
by the load current change plus any output voltage ripple
must be within the voltage tolerance of the load (CPU).
The required ESR due to a load current step is:
RESR < ∆V/∆I
where ∆I is the change in current from full load to zero load
(or minimum load) and ∆V is the allowed voltage deviation
(not including any droop due to finite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be sufficient to absorb the change in inductor current
when a high current to low current transition occurs. The
opposite load current transition is generally determined by
the control loop OPTI-LOOP components, so make sure
not to over compensate and slow down the response. The
minimum capacitance to assure the inductors’ energy is
adequately absorbed is:
( )2
L ∆I
( ) COUT >
2 ∆V VOUT
where ∆I is the change in load current.
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