6

Application Information

erated by the ESR of the output capacitors. This ramp is

proportional to the AC current through the main inductor

and is offset by the value of the DC output voltage. This

control scheme inherently compensates for variation in

either line or load conditions, since the ramp signal is gen-

erated from the output voltage itself. This control scheme

differs from traditional techniques such as voltage mode,

which generates an artificial ramp, and current mode,

which generates a ramp from inductor current.

put voltage is used to generate both the error signal and

the ramp signal. Since the ramp signal is simply the output

voltage, it is affected by any change in the output regard-

less of the origin of that change. The ramp signal also con-

tains the DC portion of the output voltage, which allows

the control circuit to drive the main switch to 0% or 100%

duty cycle as required.

Reference

Voltage

C

–

E

+

–

Ramp Signal

Output

Voltage

Feedback

Error

Signal

GATE(H)

GATE(L)

Error

Amplifier

COMP

PWM

Comparator

Theory Of Operation

Typical Performance Characteristics

150

125

100

75

50

25

0

0

2000

4000

6000

8000

10000

12000

14000

16000

Load Capacitance (pF)

Figure 4: Percent Output Error vs. DAC Output

Figure 1: Gate(H) and Gate(L) Falltime vs. Load Capacitance.

Figure 2: Gate(H) and Gate(L) Risetime vs. Load Capacitance.

Figure 3: DAC Output Voltage vs. Temperature,

DAC Code = 00001.

0.10

0.05

0

−0.05

−0.10

−0.15

−0.20

1.325 1.375 1.425 1.475 1.525 1.575 1.625 1.675 1.725 1.775 1.825 1.875 1.925 1.975 2.025 2.075

DAC Output Voltage Setting (V)

Figure 5: Percent Output Error vs. DAC Output

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

−0.05

−0.10

−0.15

−0.20

−0.25

2.125 2.225 2.325 2.425 2.525 2.625 2.725 2.825 2.925 3.025 3.125 3.225 3.335 3.425 3.525

DAC Output Voltage Setting (V)

150

125

100

75

50

25

0

0

2000

4000

6000

8000

10000

12000

14000

16000

Load Capacitance (pF)

0.10

0.05

0

−0.05

−0.10

−0.15

0

20

40

60

80

100

120

Junction Temperature (°C)