ML4813

11

sin

2

1

k

r

k

r

p

=

Ê

(kHz)

r

20

167

9.1

30

250

11.2

40

333

12.9

50

417

14.4

60

500

25.8

70

583

17.1

80

667

18.3

90

750

19.4

100

833

20.4

110

917

21.4

120

1000

22.4

130

1083

23.3

140

1167

24.2

150

1250

25.0

160

1333

25.7

170

1417

26.5

180

1500

27.3

190

1583

28.0

200

1667

28.9

The output voltage "rides" on the input voltage when the

(+) output is measured with respect to PWR GND as

shown in Figure 10.

The extra op amp provided in the ML4813 can be used to

sense the output voltage for regulation and overvoltage

conditions. This op amp is connected as a difference

amplifier with its output referenced to PWR GND.

Resistors RH1, RH2, RL1, RL2 are used to scale down the

voltage.

Normally, RH1 = RH2 = RH and RL1 = RL2 = RL. The

VV

RL

RH RL

SOUT

=™

+

(16)

The output capacitance should be calculated such that it

has the required output ripple at the worst case operating

point. In addition, the ESR should be sufficiently low to

prevent excessive dissipation due to RMS currents. The

first criterion can be met by choosing the value of the

output capacitor based on the following:

C

P

fV

V

OUT

IN

LR

OUT

˜

™™

2

pD

(17)

Where:

The second criterion for the selection of the output

capacitor can be satisfied by choosing a component with

adequately low ESR value that can safely bypass the RMS

currents.

OUTPUT DIODE

The output diode can be a "fast" or ultrafast' type

depending on the operating frequency. Reverse recovery

losses are low since under normal operating conditions,

the regulator operates in discontinuous current mode. The

diode should be rated to handle the maximum output

current. The resulting power dissipation will be the

forward drop of the diode times the output current.

POWER SWITCH

If a power MOSFET is used, it should be sized for the

lower losses, but if they are operated at high frequencies

(100kHz), higher charge dumping losses will be

experienced. The RMS current value through the power

FET and the sensing resistor is:

I

LI

f

V

k

r

RMS

PL

RMS

k

r

=

™™

™

™

=

Ê

3

2

1

424

.

sin

p

(15)

Where:

Table 1 is provided to assist in calculating (18). When the

then the power dissipation produced can be calculated

by:

P

P

V

V

D

IN

RMS

CE

=

™

™

09

.

(19)

Where:

Figure 10. Output Voltage with Respect to PWR GND

TIME

PWR GND

200V

VOUT

VOUT+

VOUT-

Table 1. Constants for Calculating IRMS (Equation 18)