LTC1626CS datasheet(7/12 Pages) LINER

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LTC1626

sequence, the LTC1626 will delay entering Burst Mode

operation and efficiency will be degraded at low currents.

APPLICATIONS INFORMATION

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:

RMS

OUT

≈

−

()

[] ()

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 0.1

µFceramiccapacitorisalsorequiredonPWRVIN

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 M

µ is a registered trademark of Magnetics, Inc.

INPUT VOLTAGE (V)

200

180

160

140

120

100

1626 F03

5.0 5.5 6.0 6.5 7.0 7.5

4.5

2.5

3.5 4.0

3.0

FIGURE 1 CIRCUIT

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 M

µ® willallowtheuser

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

Figure 3. Operating Frequency vs Supply Voltage

for Circuit Shown in Figure 1

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