UCC1858 Unitrode Semiconductor, UCC1858 Datasheet - Page 9

• Manufacturer Part Number: UCC1858
• Description: High Efficiency, High Power Factor Preregulator
• Manufacturer: Unitrode Semiconductor

APPLICATION INFORMATION (cont.)
load operation or the feedforward be derived directly from
the AC side of the input bridge as shown in the typical ap-
plications 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 characteristic
curve in Figure 7 for determining the required external re-
sistance.
Current Amplifier Set-up
The multiplier is set-up first by choosing the V
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.
Figure 7. Reguired Series Gate Resistance as a
40
30
20
10
0
10
R
MULT
Function of Supply Voltage
I
Lpk
12
I
MULTpk
R
SENSE
VDD(V)
14
16
RMS
range.
18
(5)
9
The peak current limiting function provided by the
UCC3858 is integrated into MOUT. The signal on MOUT
is normally maintained at 0V as the I
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-
cel the voltage drop across R
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:
The current amplifier can be compensated using pre-
viously presented techniques, (Application Note U- 134),
summarized here. A simplified high frequency model for
inductor current to duty cycle transfer function is given
by:
The gain of the current feedback path at the frequency of
interest (crossover) is given by:
Where V
UCC3858. Combining equations 7 and 8 yields the loop
gain of the current loop and equating it to 1 at the desired
crossover frequency can result in a design value for R
The current loop crossover frequency selected using con-
ventional trade offs. However, it should be ensured that
the current-loop is stable at the minimum switching fre-
quency under foldback conditions.
Voltage Amplifier Set-up
The voltage amplifier in UCC3858 is a transconductance
type amplifier to allow output voltage monitoring for an
overvoltage condition. The gain of the amplifier, given by
G
ation of the output second harmonic ripple (as described
in previous Unitrode Application Notes - U-134, U-159)
while providing the best transient response.
M
Z
V
I
G
D
I
LIM
L
SE
, should be chosen to achieve desired attenu-
ID
is the ramp amplitude (p-p) which is 3.5V for
s
R
I
SENSE
MULT
D
I
L
R
V
R
S
SENSE
R
L
R
O
MULT
Z
I
V
1
SE
0.5
SENSE
MULT
and the voltage at
UCC1858
UCC2858
UCC3858
R
MULT
can-
(6)
(7)
(8)
Z
.