ISL6537A Datasheet, PDF (11/16 Pages) Intersil Corporation – ACPI Regulator/Controller for Dual Channel DDR Memory Systems

ISL6537A
diagrams on page 4). 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 its 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)
(EQ. 3)
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_O
U
T
(M
A
X
)
+
(_---Δ_----I_---)
2
,
where ΔI 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 ISL6537A IC junction temperature reaches a nominal
temperature of +140°C, all regulators will be disabled. The
ISL6537A 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 ISL6537A 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 ISL6537A
switching converter. The switching components are the most
critical because they switch large amounts of energy, and
therefore tend to generate large amounts of noise. Next are
the small signal components which connect to sensitive
nodes or supply critical bypass current and signal coupling.
A multi-layer printed circuit board is recommended. Figure 2
shows the connections of the critical components in the
converter. Note that capacitors C_IN and C_OUT could each
represent numerous physical capacitors. Dedicate one solid
layer, usually a middle layer of the PC board, for a ground
plane and make all critical component ground connections
with vias to this layer. Dedicate another solid layer as a
power plane and break this plane into smaller islands of
common voltage levels. Keep the metal runs from the
PHASE terminals to the output inductor short. The power
plane should support the input power and output power
nodes. Use copper filled polygons on the top and bottom
circuit layers for the phase nodes. Use the remaining printed
circuit layers for small signal wiring. The wiring traces from
the GATE pins to the MOSFET gates should be kept short
and wide enough to easily handle the 1A of drive current.
In order to dissipate heat generated by the internal V_TT
LDO, the ground pad, pin 29, should be connected to the
internal ground plane through at least four vias. This allows
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