ADDC02808PB

REV. A

–11–

Even when the load current steps down to 0.1 A, the maximum

deviation of the output voltage is only about 400 mV, or 5%.

However, it is important to realize that if the next power pulse

occurs before this transient is over, then the output voltage will

not have the response depicted in the last section. This is be-

cause the feedback integrator will not have had time to return

to its normal state, and so the converter’s ability to respond to a

positive step change in load current will be reduced. The maxi-

mum negative going deviation in the output voltage under this

circumstance will then be greater than is shown in the figures of

that section.

Should this situation arise, one approach would be to step the

load current down to an intermediate value (e.g., 4 A) at the

end of the power pulse, and then let this current decay to a

smaller value (e.g., 0.1 A) with a time constant in the 100

µs to

200

µs range. This should permit a rapid return to a steady state

condition at the end of the power pulse without requiring a large

average load current during the low power portion of the cycle.

PIN CONNECTIONS

Pins 1 and 2 ( SENSE)

Pins 1 and 2 must always be connected for proper operation,

although failure to make these connections will not be cata-

strophic to the converter under normal operating conditions.

Pin 1 must always be connected to the output return and Pin 2

be made at any one of the output pins of the converter, or

remotely at the load. A remote connection at the load can

adjust for voltage drops of as much as 0.25 V dc between the

converter and the load.

Long remote sense leads can affect converter stability, although

this condition is rare. The impedance of the long power leads

between the converter and the remote sense point could affect

the converter’s unity gain crossover frequency and phase margin.

Consult factory if long remote sense leads are to be used.

Pin 3 (ADJUST)

An adjustment pin is provided so that the user can change the

nominal output voltage during the prototype stage. Since very

low temperature coefficient resistors are used to set the output

voltage and maintain tight regulation over temperature, using

standard external resistors to adjust the output voltage will

loosen output regulation over temperature. Furthermore, since

the status trip point is not changed when the output voltage is

adjusted using external resistors, the status line will no longer

trip at the standard levels of the newly adjusted output voltage.

If necessary, modified standard units can be ordered with the

necessary changes made inside the package at the factory. The

ADJUST function is sensitive to noise, and care should be

taken in the routing of connections.

To make the output voltage higher, place a resistor from AD-

JUST (Pin 3) to –SENSE (Pin 1). To make the output voltage

lower, place a resistor from ADJUST (Pin 3) to +SENSE (Pin

2). Figures 30 and 31 show resistor values for a

±5% change in

output voltage.

With regard to the range that the output voltage can be adjusted

by the user, there are two concerns. As the output voltage is

raised it may become difficult to maintain regulation at full

power and low input voltage. As the output voltage is lowered,

it may become difficult to maintain regulation at minimum

power and high input line.

OUTPUT VOLTAGE – %

8

7

1

99

95

98

97

96

5

4

3

2

6

Figure 30. External Resistor Value for Reducing Output

Voltage

OUTPUT VOLTAGE – %

5

4

0

101

105

102

103

104

3

2

1

Figure 31. External Resistor for Increasing Output Voltage

Pin 4 (STATUS)

Pin 4 is active high referenced to –SENSE (Pin 1), indicating

that the output voltage is typically within

±5%. The pin is both

pulled up and down by internal circuitry. Figures 32 and 33

show the typical source and sink capabilities of the status out-

put. Refer to the paragraphs describing Pin 3 (ADJUST) for

effect on status trip point.

5

4

0

0.2

1.6

0.4

0.6

0.8

1.0

1.2

1.4

3

2

1

Figure 32. Source Capability of Status Output