ISL6740A Datasheet, PDF (11/15 Pages) Intersil Corporation – Flexible Double-Ended Voltage-Mode PWM Controller with Voltage Feed Forward

ISL6740A
UV/Inhibit fault; otherwise, it is inactive and does not affect
the node 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. A small capacitor
placed between the UV input and ground may be required to
filter noise out.
V_IN
R1
R3
R2
1.00V +
-
10µA
ON
FIGURE 5. UV HYSTERESIS
As V_IN decreases to a UV condition, the threshold level is:
V_IN(DOWN)
=
R_-----1_-----+-----R_-----2_--
R2
V
(EQ. 7)
The hysteresis voltage, ∆V, is:
∆V
=
10^–5 •
〈
R1
+
R3
•


R_-----1_{---R--+---2--}R_-----2_{--}
〉
V
(EQ. 8)
Setting R3 equal to zero results in the minimum hysteresis,
and yields:
∆V = 10^–5 • R1
V
(EQ. 9)
As V_IN increases from a UV condition, the threshold level is:
V_IN(UP) = V_IN(DOWN) + ∆V
V
(EQ. 10)
Output voltage variation caused by changes in the supply
voltage may be virtually removed through a technique known
as feed forward compensation. Using feed forward, the duty
cycle is directly modulated based on changes in the input
voltage only. No closed loop feedback system is required.
The feed forward circuit uses the voltage applied to the
UV/FF pin to modulate the oscillator ramp amplitude with
minimal effect on the switching frequency and deadtime of
the oscillator. The voltage feed forward operates over a 3:1
input voltage range.
V_UV/FF
V_ERROR
CT
OUTA
OUTB
FIGURE 6. FEED FORWARD BEHAVIOR
The voltage applied to the UV/FF pin is multiplied by 0.8 and
output on the R_TC and R_TD pins. This voltage is also
summed with the C_T valley threshold voltage (0.8 V) to
create the C_T peak threshold voltage. As the voltage applied
to UV/FF varies, the CT peak voltage and the CT charge and
discharge currents vary, all in direct proportion to each other.
The result is an amplitude modulated sawtooth waveform on
C_T that is frequency invariant.
The voltage amplitude of C_T ranges from 1.6V to 4.2V as the
voltage on UV increases. The UV threshold defines the
minimum amplitude of C_T and corresponds to maximum duty
cycle operation.
For unregulated bus converters and DC transformers, feed
forward can compensate for input voltage variations without
a closed loop feedback network. A resistive voltage divider
from V_REF to V_ERROR sets the feed forward control voltage.
For example, if the desired duty cycle at the minimum
operating voltage is 90%, then
V_ERROR = D_max(V_{UV ⁄ FF} • 0.8) + 0.8
V
= 0.9(1.0 • 0.8) + 0.8 = 1.52
V
(EQ. 11)
Overcurrent Protection
There are two overcurrent protection mechanisms in the
ISL6740A, one for light overcurrent and one for heavy over
load. They are referred to, respectively, as overcurrent
protection and short circuit protection.
Overcurrent Operation
Overcurrent delayed shutdown is enabled once the soft-start
cycle is complete. If an overcurrent condition is detected, the
soft-start charging current source is disabled and the soft-
start capacitor is allowed to discharge through a 15µA
source. At the same time a 50µs re-triggerable one-shot
timer is activated. It remains active for 50µs after the
overcurrent condition ceases. If the soft-start capacitor
discharges by more then 0.25V to 4.25V, the output is
disabled and the Fault signal asserted. This state continues
until the soft-start voltage reaches 270mV, at which time a
new soft-start cycle is initiated. If the overcurrent condition
11
FN9195.0
February 7, 2005