ISL6521 Просмотр технического описания (PDF) - Renesas Electronics
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ISL6521 Datasheet PDF : 14 Pages
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ISL6521
used in parallel with ROCSET (1nF recommended). Upon turn-
off of the pull-down device, the switching regulator undergoes
a soft-start cycle.
To disable a particular linear controller, pull and hold the
respective FB pin above a typical threshold of 1.25V. One way
to achieve this task is by using a logic gate coupled through a
small-signal diode. The diode should be placed as close to the
FB pin as possible to minimize stray capacitance to this pin.
Upon turn-off of the pull-up device, the respective output
undergoes a soft-start cycle, bringing the output within
regulation limits. On regulators implementing this feature, the
parallel combination of the feedback resistors has to be
sufficiently high to allow ease of driving from the external
device. Considering the other restriction applying to the upper
range of this resistor combination (see ‘Output Voltage
Selection’ paragraph), it is recommended the values of the
feedback resistors on the linear regulator output meet the
following constraint:
2k -RR----SS-----+-----RR----P-P- 5k
Important Note When Using External Pass Devices
If the collector voltage to a linear regulator pass transistor (Q3,
Q4, or Q5 shown in Figure 7) is lost, the respective regulator
has to be shut down by pulling high its FB pin. This measure is
necessary in order to avoid possible damage to the ISL6521 as
a result of overheating. Overheating can occur in such
situations due to sheer power dissipation inside the chip’s
linear drivers.
Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. The voltage
spikes can degrade efficiency, radiate noise into the circuit,
and lead to device overvoltage stress. Careful component
layout and printed circuit design minimizes the voltage spikes
in the converter. Consider, as an example, the turn-off
transition of the upper PWM MOSFET. Prior to turn-off, the
upper MOSFET was carrying the full load current. During the
turn-off, current stops flowing in the upper MOSFET and is
picked up by the lower MOSFET or Schottky diode. Any
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 circuit traces minimize the magnitude of
voltage spikes.
There are two sets of critical components in a DC-DC
converter using an ISL6521 controller. The switching power
components are the most critical because they switch large
amounts of energy, and as such, they tend to generate equally
large amounts of noise. The critical small signal components
are those connected to sensitive nodes or those supplying
critical bypass current.
The power components and the controller IC should be placed
first. Locate the input capacitors, especially the high-frequency
ceramic decoupling capacitors, close to the power switches.
Locate the output inductor and output capacitors between the
MOSFETs and the load. Locate the PWM controller close to
the MOSFETs.
The critical small signal components include the bypass
capacitor for VCC and the feedback resistors. Locate these
components close to their connecting pins on the control IC.
A multi-layer printed circuit board is recommended. Figure 7
shows the connections of the critical components in the
converter. Note that the capacitors CIN and COUT each can
represent numerous physical capacitors. Dedicate one solid
layer 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. 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, but do not unnecessarily oversize these
particular islands. Since the PHASE nodes are subjected to
very high dv/dt voltages, the stray capacitor formed between
these islands and the surrounding circuitry will tend to couple
switching noise. Use the remaining printed circuit layers for
small signal wiring. The wiring traces from the control IC to
the MOSFET gate and source should be sized to carry 2A
peak currents.
FN9148 Rev 2.00
Feb 8, 2005
Page 9 of 14
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