isl9444 Intersil Corporation, isl9444 Datasheet - Page 19

• Manufacturer Part Number: isl9444
• Manufacturer: Intersil Corporation

circumstances are expected, an additional external Schottky
diode may be added from VCC_5V to the positive of the boot
capacitor. RBOOT may still be necessary to lower EMI due to fast
turn-on of the upper MOSFET.
Power-Good Indicators
The three independent Power-good pins can be used to monitor
the status of the output voltages. PGOODx will be true (open
drain) when the corresponding FBx pin is within ±11% of the
reference voltage.
Additionally, a capacitor from the PG3_DLY pin to the ground sets
a delay time for the PGOOD3 signal. After FB3 pin enters ±11%
of the reference range, a 1.9µA current begins charging the CDLY
capacitor. When the PG3_DLY voltage reaches 1.2V PGOOD3
goes HIGH.
The typical delay time is set according to Equation 8:
There is no extra delay when the PGOOD3 pin is pulled LOW.
Protection Circuits
The converter outputs are monitored and protected against
overload, short circuit and undervoltage conditions.
Undervoltage Lockout
The ISL9444 includes UVLO protection which keeps the device in
a reset condition until a proper operating voltage is applied. It
also shuts down the ISL9444 if the operating voltage drops
below a pre-defined value. All controllers are disabled when UVLO
is asserted. When UVLO is asserted, PGOOD1, PGOOD2 and
PGOOD3 are valid and will be de-asserted.
Overcurrent Protection
All the PWM controllers use the lower MOSFET's on-resistance,
r
voltage drop is compared with a threshold set by a resistor
connected from the OCSETx pin to ground.
R
Where I
R
ISENx pin.
If an overcurrent is detected, the upper MOSFET remains off and
the lower MOSFET remains on until the next cycle. As a result, the
converter will skip a pulse. When the overload condition is
removed, the converter will resume normal operation.
If an overcurrent is detected for 2 consecutive clock cycles, the IC
enters a hiccup mode by turning off the gate drivers and entering
soft-start. The IC will cycle 5 times through soft-start before trying to
restart. The IC will continue to cycle through soft-start until the
overcurrent condition is removed. Hiccup mode is active during
soft-start so care must be taken to ensure that the peak inductor
current does not exceed the overcurrent threshold during soft-start.
t
DS(ON)
DLY
OCSET
CS
is a value of the current sense resistor connected to the
=
, to monitor the current in the converter. The sensed
1.2V
OC
=
-------------------------------------- -
(
is the desired overcurrent protection threshold, and
I
OC
⎛
⎝
--------------- -
1.9μA
7 ( ) R
C
) r
DLY
(
(
DS ON
CS
⎞
⎠
(
)
)
)
19
(EQ. 8)
(EQ. 9)
ISL9444
Because of the nature of this current sensing technique, and to
accommodate a wide range of r
overcurrent threshold should represent an overload current about
150% to 180% of the maximum operating current. If more
accurate current protection is desired, place a current sense
resistor in series with the lower MOSFET source.
When OCP is triggered, the EN/SS1 or TK/SSx pins are pulled to
ground by an internal MOSFET. For PWM rails configured to track
another voltage rail, the TK/SSx pin rises up much faster than
the internal minimum soft-start ramp. The voltage reference will
then be clamped to the internal minimum soft-start ramp. Thus
smooth soft-start hiccup is achieved even with tracking function.
Overvoltage Protection
All switching controllers within the ISL9444 have fixed
overvoltage set points. The overvoltage set point is set at 118%
of the nominal output voltage, the output voltage set by the
feedback resistors. In the case of an overvoltage event, the IC will
attempt to bring the output voltage back into regulation by
keeping the upper MOSFET turned off and modulating the lower
MOSFET for 2 consecutive PWM cycles. If the overvoltage
condition has not been corrected in 2 cycles and the output
voltage is above 118% of the nominal output voltage, the
ISL9444 will turn off both the upper MOSFET and the lower
MOSFET. The ISL9444 will enter hiccup mode until the output
voltage returns to 110% of the nominal output voltage.
Over-Temperature Protection
The IC incorporates an over-temperature protection circuit that
shuts the IC down when a die temperature of +150°C is
reached. Normal operation resumes when the die temperatures
drops below +130°C through the initiation of a full soft-start
cycle. When all three channels are disabled, thermal protection
is inactive. This helps achieve a very low shutdown current of
33µA.
Feedback Loop Compensation
To reduce the number of external components and to simplify the
process of determining compensation components, all PWM
controllers have internally compensated error amplifiers. To
make internal compensation possible, several design measures
were taken.
Firstly, the ramp signal applied to the PWM comparator is
proportional to the input voltage provided at the VIN pin. This
keeps the modulator gain constant with varying input voltages.
Secondly, the load current proportional signal is derived from the
voltage drop across the lower MOSFET during the PWM time
interval and is subtracted from the amplified error signal on the
comparator input. This creates an internal current control loop.
The resistor connected to the ISEN pin sets the gain in the current
feedback loop. The following expression estimates the required
value of the current sense resistor depending on the maximum
operating load current and the value of the MOSFET’s r
R
Choosing R
and hold circuitry is recommended but values down to 2µA and
CS
≥
(
------------------------------------------- -
I
MAX
30μA
CS
) r
(
DS ON
to provide 30µA of current to the current sample
(
)
)
DS(ON)
variations, the value of the
May 23, 2011
DS(ON)
FN7665.0
(EQ. 10)
.