UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

10

POST OFFICE BOX 655303

APPLICATION INFORMATION

PFM mode (continued)

of the current-sense resistor. Note that in this case, the PFM pin voltage is set by the R1/R2 resistive divider

off of the FB pin, which is regulated to 1.25 V.

During sleep mode, the COMP pin is forced to 70 mV above the PFM pin voltage. This minimizes error amplifier

overshoot when coming out of sleep mode, and prevents erroneously tripping the PFM comparator.

disabling PFM mode

The user may disable PFM mode by pulling the PFM pin below 0.2 V. In this case, the UCC39421 remains on,

in fixed frequency operation at all load currents. The PFM pin can also be driven, through a resistive divider,

off of an output from the system controller. This allows the system controller to prepare for an expected step

increase in load, improving the converter’s large signal transient response. An example of this is shown in

Figure 3.

UCC39421

R2

R1

PFM

ENABLE OUTPUT

FROM CONTROLLER

7

Figure 3. Driving the PFM Pin From a Controller Output

choosing a topology and optimal synchronous rectifier

The UCC39421 is designed to be very flexible, and can be used in boost, flyback and SEPIC topologies. It can

operate from input voltages between 1.8 V and 8.0 V. Output voltages can be between 2.5 V and 8.0 V. It can

also drive either N- or P-channel MOSFET synchronous rectifiers. Table 1 can be used to select the appropriate

topology for a given combination of input and output voltage requirements. Although it is designed to operate

as a peak current mode controller, it can also be configured for voltage mode control. This is discussed in a later

section.

The user can program the gate drive output on the RECT pin for N-channel MOSFETs by grounding the RSEL

pin, or for P-channel MOSFETs by connecting the RESEL pin to VIN. Table 2 is used to determine whether an

N- or P-channel synchronous rectifier should be used.

Note: In all cases, low-voltage-logic MOSFETs should be used to achieve the lowest possible on-resistance

for the highest efficiency.

The application diagrams in Figures 4 through 8 illustrate the use of the UCC39421 in all the topologies, using

N- and P-channel rectifiers. They are be discussed in detail in the next section.