ISL6534 Datasheet, PDF (21/26 Pages) Intersil Corporation – Dual PWM with Linear

ISL6534
and slow the current load rate seen by the bulk capacitors.
The bulk filter capacitor values are generally determined by
the ESR (effective series resistance) and voltage rating
requirements rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements. And keep in mind that not all
applications have the same requirements; some may need
many ceramic capacitors in parallel; others may need only one.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR will determine the output ripple
voltage and the initial voltage drop after a high slew-rate
transient. An aluminum electrolytic capacitor's ESR value is
related to the case size with lower ESR available in larger
case sizes. However, the equivalent series inductance
(ESL) of these capacitors increases with case size and can
reduce the usefulness of the capacitor to high slew-rate
transient loading. Unfortunately, ESL is not a specified
parameter. Work with your capacitor supplier and measure
the capacitor’s impedance with frequency to select a
suitable component. In most cases, multiple electrolytic
capacitors of small case size perform better than a single
large case capacitor.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors for high frequency decoupling and bulk capacitors
to supply the current needed each time Q1 turns on. Place
the small ceramic capacitors physically close to the
MOSFETs and between the drain of Q1 and the source of Q2.
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum
input voltage and a voltage rating of 1.5 times is a
conservative guideline. The RMS current rating requirement
for the input capacitor of a buck regulator is approximately
1/2 the DC load current.
For both through-hole and surface-mount design, several
electrolytic capacitors (Panasonic HFQ series or Nichicon
PL series or Sanyo MV-GX or equivalent) may be needed.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the
capacitor surge current rating. These capacitors must be
capable of handling the surge-current at power-up. The TPS
series available from AVX, and the 593D series from
Sprague are both surge current tested.
Snubbers
A snubber network is a series resistor and capacitor, usually
from the phase node to GND (across the lower FET); it is
used to dampen the ringing of the phase node, which can
introduce noise into other parts of the circuit. In particular,
jitter on the gate drivers can be caused by disturbances that
trigger the programmable duty cycle edge of the internal
ramp generator. If noise or ringing is a problem in your
particular circuit, consider adding a snubber. Typical values
are 2.2nF for the capacitor, and 2.2Ω for the resistor. Note
that the resistor may have large currents, so use a 1/2W type
resistor. The order of R and C doesn’t usually matter, but one
preference is putting the resistor to GND, such that the
voltage across it can be easily measured on an oscilloscope
to represent the current. See Figure 19.
VIN1
UGATE1
PHASE1
LGATE1
VOUT1
CSN1
RSN1
FIGURE 19. SNUBBER COMPONENT SELECTION
Optional Schottky Selection
An optional rectifier D2 (see Fig 17 or 18) is a clamp that
catches the negative inductor swing during the dead time
between turning off the lower MOSFET and turning on the
upper MOSFET. The diode must be a Schottky type to prevent
the lossy parasitic MOSFET body diode from conducting. If
used, connect the cathode to the phase node, and the anode
to PGND. It is acceptable to omit the diode and let the body
diode of the lower MOSFET clamp the negative inductor
swing, but efficiency will drop one or two percent as a result.
The diode's rated reverse breakdown voltage must be greater
than the maximum input voltage.
Margining and “Fine-Tuning”
Margining can be added externally to a voltage regulator, in
order to raise and/or lower the output voltage a nominal
amount, such as ±10%. The purpose might be to run the
processor at higher voltage for faster clock speeds, or to run
at lower voltages, to save power, for example.
A straightforward method involves adding two extra resistors
and two small FETs (and re-adjusting R2, depending upon
the decoding used); see Figure 20. Both resistors (RM1,
RM2) are high values (10-100kΩ) compared to R1 and R2
(~1kΩ). So when placed in parallel with R2, it lowers the
resistance of R2; pick the values for the desired amount.
Some simple logic is needed on the gates A and B to control
them; pull-up or pull-down resistors might also be needed.
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FN9134.1