ISL6548 Datasheet, PDF (10/15 Pages) Intersil Corporation – ACPI Regulator/Controller for Dual Channel DDR Memory Systems

ISL6548
will immediately shut down when the Fault Counter reaches
a count of 5 at any other time.
The 16384 counts that are required to reset the Fault Reset
Counter represent 8 soft-start cycles, as one soft-start cycle
is 2048 clock cycles. This allows the ISL6548 to attempt at
least one full soft-start sequence to restart the faulted
regulators.
When attempting to restart a faulted regulator, the ISL6548
will follow the preset start up sequencing. If a regulator is
already in regulation, then it will not be affected by the start
up sequencing.
V_DDQ Overcurrent Protection
The overcurrent function protects the switching converter from
a shorted output by using the upper MOSFET on-resistance,
r_DS(ON), to monitor the current. This method enhances the
converter’s efficiency and reduces cost by eliminating a
current sensing resistor.
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OCSET)
programs the overcurrent trip level (see Typical Application
diagrams on page 3). An internal 20µA (typical) current sink
develops a voltage across R_OCSET that is referenced to the
converter input voltage. When the voltage across the upper
MOSFET (also referenced to the converter input voltage)
exceeds the voltage across R_OCSET, the overcurrent function
initiates a soft-start sequence. The initiation of soft-start may
affect other regulators. The V_{TT_DDR} regulator is directly
affected as it receives it’s reference and input from V_DDQ.
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by:
I_PEAK
=
I--_{O-----C----S----E----T-----x-----R-----O----C-----S----E---T--}
r_DS(ON)
where I_OCSET is the internal OCSET current source (20µA
typical). The OC trip point varies mainly due to the MOSFET
r_DS(ON) variations. To avoid overcurrent tripping in the
normal operating load range, find the R_OCSET resistor from
the equation above with:
1. The maximum r_DS(ON) at the highest junction
temperature.
2. The minimum I_OCSET from the specification table.
3. Determine I_PEAK for
I_PEAK
>
I_OUT(MAX)
+
(_---∆_----I_---)
2
,
where ∆I is the output inductor ripple current.
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
A small ceramic capacitor should be placed in parallel with
R_OCSET to smooth the voltage across R_OCSET in the
presence of switching noise on the input voltage.
Thermal Protection (S0/S3 State)
If the ISL6548 IC junction temperature reaches a nominal
temperature of 140°C, all regulators will be disabled. The
ISL6548 will not re-enable the outputs until the junction
temperature drops below 110°C and either the bias voltage is
toggled in order to initiate a POR or the SLP_S5 signal is
forced LOW and then back to HIGH.
Shoot-Through Protection
A shoot-through condition occurs when both the upper and
lower MOSFETs are turned on simultaneously, effectively
shorting the input voltage to ground. To protect from a shoot-
through condition, the ISL6548 incorporates specialized
circuitry on the V_DDQ regulator which insures that
complementary MOSFETs are not ON simultaneously.
The adaptive shoot-through protection utilized by the V_DDQ
regulator looks at the lower gate drive pin, LGATE, and the
upper gate drive pin, UGATE, to determine whether a
MOSFET is ON or OFF. If the voltage from UGATE or from
LGATE to GND is less than 0.8V, then the respective
MOSFET is defined as being OFF and the other MOSFET is
allowed to turned ON. This method allows the V_DDQ
regulator to both source and sink current.
Since the voltage of the MOSFET gates are being measured
to determine the state of the MOSFET, the designer is
encouraged to consider the repercussions of introducing
external components between the gate drivers and their
respective MOSFET gates before actually implementing
such measures. Doing so may interfere with the shoot-
through protection.
Application Guidelines
Layout Considerations
Layout is very important in high frequency switching
converter design. With power devices switching efficiently at
250kHz, the resulting current transitions from one device to
another cause voltage spikes across the interconnecting
impedances and parasitic circuit elements. These voltage
spikes can degrade efficiency, radiate noise into the circuit,
and lead to device overvoltage stress. Careful component
layout and printed circuit board design minimizes these
voltage spikes.
As an example, consider the turn-off transition of the control
MOSFET. Prior to turn-off, the MOSFET is carrying the full
load current. During turn-off, current stops flowing in the
MOSFET and is picked up by the lower MOSFET. Any
parasitic inductance in the switched current path generates a
large voltage spike during the switching interval. Careful
component selection, tight layout of the critical components,
and short, wide traces minimizes the magnitude of voltage
spikes.
There are two sets of critical components in the ISL6548
switching converter. The switching components are the most
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FN9188.1
February 9, 2005