ISL6549 INTERSIL [Intersil Corporation], ISL6549 Datasheet - Page 8

• Manufacturer Part Number: ISL6549
• Description: Single 12V Input Supply Dual Regulator Synchronous Rectified Buck PWM and Linear Power Controller
• Manufacturer: INTERSIL [Intersil Corporation]

.
A few clock cycles are used for initialization to insure that soft-
start begins near zero volts. The ramps are the same, whether
triggered by releasing FS_DIS or by exceeding the POR trip
levels.
Both outputs use the same soft-start ramp, and the ramp time
is determined by the switching frequency. Thus, there is no
simple way to disable or sequence them independently, or to
change the ramp rate independently of the clock.
If the switcher output is already pre-charged to a voltage when
the regulator starts up, the ISL6549 will detect this condition
(see Figure 3). The red trace shows the normal ramp, when
the output starts at GND. The green trace shows the case
when the output is pre-charged to a voltage less than the final
output. The upper or lower FET does not turn on until the soft-
start ramp voltage exceeds the output; then the output starts
ramping seamlessly from there. If the output voltage is pre-
charged above the normal output level, as shown in the
magenta trace, neither FET will turn on until the end of the
soft-start ramp; then the output will be quickly pulled down to
the final value.
GND>
GND>
GND>
GND>
GND>
GND>
FIGURE 2. EXPANDED VIEW: VOLTAGE RAMP AND TIME
V
OUT2
FIGURE 3. PRE-CHARGED OUTPUT
PRE-CHARGED (1V/DIV)
V
VCC12 > 9.5V
OUT2
NO CHARGE (1V/DIV)
V
OUT2
OVER-CHARGED (1V/DIV)
8
V
VCC12 (2V/DIV)
OUT1
V
OUT2
ISL6549
Undervoltage Protection
The FB and LDO_FB pins are each monitored during
converter operation by their own Undervoltage (UV)
comparator. If either FB voltage drops below 75% of the
reference voltage (75% of 0.8V = 0.6V), a fault signal is
internally generated, and the fault logic shuts down BOTH
regulators. The UV comparators are enabled when the
soft-start ramp is about one-quarter (25%) done.
Figure 4 illustrates the protection feature responding to a UV
event on V
of the nominal output voltage. Both outputs are quickly shut
down and the UGATE and LGATE stop switching immediately,
but the fall time of each output is determined by the load
and/or short condition on each plus the output capacitance
that needs to be discharged. The soft-start function begins
producing an internal soft-start ramp. The delay interval, T0 to
T1, seen by the output is equivalent to one soft-start cycle.
Then a normal soft-start ramp of the output starts, at time T1.
At the one-quarter point of the soft-start ramp (not drawn
exactly to scale), the good output will have ramped one-
quarter way up, while the shorted output will presumably be
lower than a quarter (depending on the magnitude of the
short). Once the UV comparators are enabled (at the
one-quarter point) both outputs will again shut down (if the
fault is still present on one of them). Time T2 starts a new
internal soft-start cycle, and at T3, starts a new ramp, similar
to T1. This time, if we assume the short has gone away, the
outputs will ramp up to T4 as they should. If the short has not
gone away, then the T0, T1, T2 hiccup mode cycle will keep
repeating indefinitely; this cycle time is the equivalent of 1.25
GND>
GND>
GND>
FIGURE 4. UNDERVOLTAGE PROTECTION RESPONSE
(0.5V/DIV)
(T1 TO T2 NOT TO SCALE)
INTERNAL SOFT-START FUNCTION
T0
OUT1
SOFT-START
DELAY INTERVAL
DELAY
V
. At time T0, V
OUT1
V
OUT2
(1.5V)
T1
(2.5V)
T2
SOFT-START
OUT1
DELAY
TIME
has dropped below 75%
T3
September 22, 2006
T4
FN9168.2