7

LTC1626

sequence, the LTC1626 will delay entering Burst Mode

operation and efficiency will be degraded at low currents.

the P-channel switch duty cycle. At high input voltages,

The most stressful condition for the diode is when the

regulator output is shorted to ground.

Under short-circuit conditions, the diode must safely

circuits will be well served by either an MBRM5819 or an

MBRS130LT3. An MBR0520LT1 is a good choice for

In continuous mode, the input current of the converter is

voltage transients, a low effective series resistance (ESR)

input capacitor must be used. In addition, the capacitor

given by:

I

IV

V

V

V

A

RMS

OUT

OUT

IN

OUT

IN

≈

−

()

12

/

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

for high frequency decoupling.

Kool M

µ is a registered trademark of Magnetics, Inc.

INPUT VOLTAGE (V)

200

180

160

140

120

100

80

60

40

20

0

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

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