LTC1626 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1626' PDF : 12 Pages View PDF
LTC1626
APPLICATIONS INFORMATION
sequence, the LTC1626 will delay entering Burst Mode
operation and efficiency will be degraded at low currents.
200
180 FIGURE 1 CIRCUIT
160
140
120
100
80
60
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20
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 F03
Figure 3. Operating Frequency vs Supply Voltage
for Circuit Shown in Figure 1
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor—core and copper losses.
Core losses are dependent on the peak-to-peak ripple
current and core material. However, they are independent
of the physical size of the core. By increasing inductance,
the peak-to-peak inductor ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial, such as molypermalloy or Kool Mµ® will allow the user
to concentrate on reducing copper loss and preventing
saturation.
Although higher inductance reduces core loss, it increases
copper loss as it requires more windings. When space is
not a premium, larger wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation
in the inductor.
Catch Diode Selection
Losses in the catch diode depend on forward drop and
switching times. Therefore, Schottky diodes are a good
choice for low drop and fast switching times.
The catch diode carries the load current during the off-
time. The average diode current is therefore dependent on
the P-channel switch duty cycle. At high input voltages,
the diode conducts most of the time. As VIN approaches
VOUT, the diode conducts only a small fraction of the time.
The most stressful condition for the diode is when the
regulator output is shorted to ground.
Under short-circuit conditions, the diode must safely
handle ISC(PK) at close to 100% duty cycle. Most LTC1626
circuits will be well served by either an MBRM5819 or an
MBRS130LT3. An MBR0520LT1 is a good choice for
IOUT(MAX) ≤ 500mA.
Input Capacitor (CIN) Selection
In continuous mode, the input current of the converter is
a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low effective series resistance (ESR)
input capacitor must be used. In addition, the capacitor
must handle a high RMS current. The CIN RMS current is
given by:
[ ( )] ( ) 1/2
IOUT VOUT VIN − VOUT
IRMS ≈
VIN
A
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst case is commonly used
to 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 life-
time. This make it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Do not underspecify this component. An addi-
tional 0.1µF ceramic capacitor is also required on PWR VIN
for high frequency decoupling.
Output Capacitor (COUT) Selection
The selection of COUT is driven by the ESR for proper
operation of the LTC1626. The required ESR of COUT is:
ESRCOUT < 50mV/IRIPPLE
where IRIPPLE is the ripple current of the inductor. For the
case where the IRIPPLE is 25mV/RSENSE, the required ESR
of COUT is:
Kool Mµ is a registered trademark of Magnetics, Inc.
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