ISL6726_14 Datasheet, PDF (18/20 Pages) Intersil Corporation – Active Clamp Forward PWM Controller

ISL6726
Configuring UV
The UV input is used for input source undervoltage lockout. If the
UV node voltage falls below 1.00V, a UV shutdown fault occurs.
This may be caused by low source voltage or by intentional
grounding of the pin to disable the outputs. There is a nominal
10µA switched current source used to create hysteresis. The
current source is active only during an UV/Inhibit fault; otherwise,
it is inactive and does not affect the UV threshold voltage. The
magnitude of the hysteresis is a function of the external resistor
divider impedance. If the resistor divider impedance results in too
little hysteresis, a series resistor between the UV pin and the
divider may be used to increase the hysteresis. A soft-start cycle
begins when the UV/Inhibit fault clears. The voltage hysteresis
created by the switched current source and the external
impedance is generally small due to the large resistor divider ratio
required to scale the input voltage down to the UV threshold level.
V_IN
R1
R3
R2
1.00V +
-
10µA
ON
FIGURE 17. UV HYSTERESIS
Referring to Figure 17, as V_IN decreases to a UV condition, the
threshold level is:
V_IN(DOWN)
=
R_-----1_-----+-----R_----2_--
R2
V
(EQ. 9)
The hysteresis voltage, ΔV, is:
ΔV
=
10^–5 •
〈
R
1
+
R3
•
⎛
⎝
R_-----1_{---R--+--2---}R_----2_{--⎠⎞}
〉
V
(EQ. 10)
Setting R3 equal to zero results in the minimum hysteresis, and
yields:
ΔV = 10^–5 • R1
V
(EQ. 11)
As V_IN increases from a UV condition, the threshold level is:
V_IN(UP) = V_IN(DOWN) + ΔV
V
(EQ. 12)
Although the current hysteresis provides great flexibility in setting
the magnitude of the hysteresis voltage, it is susceptible to noise
on the signal. If the hysteresis was implemented as a fixed
voltage instead, the signal could be filtered with a small
capacitor placed between the UV pin and signal ground. This
technique does not work well when the hysteresis is a current
source because a current source takes time to charge the filter
capacitor. There is no instantaneous change in the threshold
level thereby rendering the current hysteresis ineffective. To
remedy the situation the filter capacitor must be separated from
the UV pin by a resistor. Referring to Figure 17, the filter capacitor
must be placed in parallel with R2, and the capacitor and R3
must be physically close to the UV pin.
UV may also be used as an inhibit signal by externally pulling it
below the 1V threshold. However, caution must be exercised as
the maximum duty cycle limit controlled by DCLIM will be
defeated. The peak amplitude of CT will be reduced to ~1.6V
when UV decreases below the 1V turn-off threshold, and the
maximum duty cycle allowed will increase to 80%.
Slope Compensation
For applications where the maximum duty cycle is less than 50%,
slope compensation may be used to improve noise immunity,
particularly at lighter loads. The amount of slope compensation
required for noise immunity is determined empirically, but is
generally about 10% of the full scale current feedback signal.
For applications where the duty cycle is greater than 50%, slope
compensation is required to prevent instability, referred to as
sub-harmonic oscillation. Slope compensation is a technique in
which the current feedback signal is modified by adding slope,
that is, adding a linearly increasing voltage as a function of time.
The minimum amount of slope compensation required
corresponds to 1/2 the inductor downslope, as it would appear
referred to the CS input. See Figure 18. More may be added, but
increasing the slope compensation arbitrarily results in a control
loop that transitions into voltage mode as the slope
compensation begins to dominate the current feedback signal.
The minimum amount of capacitance to place at the SLOPE pin is:
C_SLOPE
=
18 • ------^t--^O----^N-------
V_SLOPE
μF
(EQ. 13)
Where t_ON is the maximum ON time in seconds, and V_SLOPE is the
amount of voltage to be added as slope compensation to the
current feedback signal at the CS pin. In general, the amount of
slope compensation added is 2 to 3 times the minimum required.
CURRENT SENSE SIGNAL ^DOWNSLOPE
TIME
FIGURE 18. DOWNSLOPE
It should be noted that the power transformer magnetizing
inductance contributes to slope compensation and should be
considered when determining the amount of slope
compensation required.
Example:
Assume the inductor current signal presented at the CS pin
decreases 125mV during the Off period, and:
Switching Frequency, F_sw = 250kHz
Duty Cycle, D = 60%
t_ON = D/F_sw = 0.6/250E3 = 2.4µs
t_OFF = (1 - D)/Fsw = 1.6µs
18
FN7654.0
January 31, 2011