UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

19

POST OFFICE BOX 655303

APPLICATION INFORMATION

PWM duty cycle and slope compensation (continued)

For a flyback topology, using a 1:1 turns ratio, the equation becomes:

R

SLOPE +

V

OUT *

V

IN min

R

SENSE

R

T

L

15)

If the converter is operated in the discontinuous conduction mode (inductor current drops to zero), no slope

compensation is required. The point at which this mode boundary occurs is a function of switching frequency,

input voltage, output voltage, load current, and inductor value. However, in general the converter is more

efficient when operated in the continuous conduction mode due to the lower peak currents.

Figure 9

CCM BOOST CONVERTER DUTY CYCLE

vs

INPUT VOLTAGE

2

0%

10%

20%

30%

40%

50%

60%

70%

34

V

V

VIN − Input Voltage − V

30%

V

V

V

40%

50%

60%

70%

80%

23

4

2.5

3.5

4.5

V

Figure 10

CCM FLYBACK CONVERTER DUTY CYCLE

vs

INPUT VOLTAGE

VIN − Input Voltage − V

voltage mode control

The UCC39421 can be operated as a voltage mode controller by connecting a 5.6-k

Ω resistor from the ISENSE

pin to ground. The internal current source generates an artificial ramp voltage on this input. In this case, no slope

compensation is required, and no current-sense resistor is required in series with the source of the N-channel

MOSFET. A typical application diagram is shown in Figure 11. However, in this configuration there is no

overcurrent protection. In addition, the pulse and low power modes, designed to increase efficiency at light

loads, operates at different load currents. This is because the internal error amplifier’s output voltage is no longer

a direct function of load current, but rather of duty cycle. When operating in CCM, the duty cycle is largely a

function of input and output voltage, not load current. At light enough loads however, the converter goes into

discontinuous mode and the error amplifier voltage drops low enough to activate the low power and pulse

modes.