10

UCC1858

UCC2858

UCC3858

The discrete nature of the RMS voltage feedforward

means that there are regions of operation where the in-

tiplier does not change. The voltage error amplifier

compensates for this by changing its output to maintain

the required multiplier output current. When the output of

the ADC changes, there is a jump in the output of the er-

ror amplifier. There is a resultant shift in the foldback fre-

quency if the converter is at light load. However, the

impact of this change is minimal on the overall converter

operation.

Another key consideration with the RMS voltage scheme

effective. At very light loads and high line conditions, the

rectified AC does not quite reach zero if a large capacitor

is being used for filtering on the rectified side of the

bridge. In such instances, the feedforward effect does not

take place and the controller functionality is lost. For

zero crossing detection to take place. It is recommended

that the capacitor value be kept low enough for the light

load operation or the feedforward be derived directly from

the AC side of the input bridge as shown in the typical

applications circuit.

Gate Drive Considerations

The gate drive circuit in UCC3858 is designed for high

speed power switch drive. It consists of low impedance

pull-up and pull-down DMOS output stages. When oper-

ating with high bias voltages, in order to stay within the

SOA of the DMOS output stages, it is recommended that

the gate drive current be limited to 0.5A peak with the

use of external gate resistor. Please see the characteris-

tic curve in Fig. 8 for determining the required external re-

sistance.

Current Amplifier Set-up

The maximum multiplier output is at low line, full load

conditions. The inductor peak current also occurs at the

same point. The multiplier terminating resistor can be de-

termined using equation 5.

R

IL

R

I

MULT

PK

SENSE

MULT PK

=

•

(5)

The peak current limiting function provided by the

UCC3858 is integrated into MOUT. The signal on MOUT

cels the voltage drop across the sense resistor with

closed loop operation. During short circuit or transient

startup conditions, the multiplier current can not fully can-

MOUT drops below 0V. The internal peak current limit is

activated when MOUT drops below –0.5V. The peak cur-

rent limit at any operating point is given by:

I

IR

R

LIM

MULT

MULT

SENSE

=

•+ 05

.

(6)

The current amplifier can be compensated using previ-

ously presented techniques, (Application Note U- 134),

summarized here. A simplified high frequency model for

inductor current to duty cycle transfer function is given

by:

Gs

i

d

V

sL

id

LO

()

=

∧

∧

=

(7)

The gain of the current feedback path at the frequency of

interest (crossover) is given by:

d

i

R

R

RV

L

SENSE

Z

ISE

∧

•

1

(8)

UCC3858. Combining equations 7 and 8 yields the loop

gain of the current loop and equating it to 1 at the de-

sired crossover frequency can result in a design value for

conventional trade offs. However, it should be ensured

that the current-loop is stable at the minimum switching

frequency under foldback conditions.

APPLICATION INFORMATION (cont.)

0

10

20

30

40

10

12

14

16

18

20

VDD(V)

Figure 8. Reguired series gate resistance as a

function of supply voltage.