ISL6223 Datasheet, PDF (14/15 Pages) Intersil Corporation – Mobile Microprocessor CORE Voltage Regulator Multi-Phase Buck PWM Controller

ISL6223
0.5
SINGLE
0.4
CHANNEL
0.3
2 CHANNEL
0.2
3 CHANNEL
0.1
4 CHANNEL
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O/V_IN)
FIGURE 14. CURRENT MULTIPLIER vs DUTY CYCLE
The current from multiple channels tend to cancel each other
and reduce the total ripple current. Figure 13 gives the total
ripple current as a function of duty cycle, normalized to the
parameter (Vo) ⁄ (L ⋅ F_S) at zero duty cycle. To determine the
total ripple current from the number of channels and the duty
cycle, multiply the y-axis value by (Vo) ⁄ (LxF_SW).
Small values of output inductance can cause excessive power
dissipation. The ISL6223 is designed for stable operation for
ripple currents up to twice the load current. However, for this
condition, the RMS current is 115% above the value shown in
the following MOSFET Selection and Considerations section.
With all else fixed, decreasing the inductance could increase
the power dissipated in the MOSFETs by 30%.
Input Capacitor Selection
The important parameters for the bulk input capacitors are
the voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors 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 required for a
multi-phase converter can be approximated with the aid of
Figure 14.
First determine the operating duty ratio as the ratio of the
output voltage divided by the input voltage. Find the Current
Multiplier from the curve with the appropriate power
channels. Multiply the current multiplier by the full load
output current. The resulting value is the RMS current rating
required by the input capacitor.
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use ceramic capacitance
for the high frequency decoupling and bulk capacitors to
supply the RMS current. Small ceramic capacitors should
be placed very close to the drain of the upper MOSFET to
suppress the voltage induced in the parasitic circuit
impedances.
For bulk capacitance, 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 or Panasonic Speciality Polymer (SP)
capacitors can be used. Caution must be exercised with regard
to the capacitor surge current rating when using the Tantalum
capacitors. 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.
MOSFET Selection and Considerations
In high-current PWM applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The power dissipation includes two
loss components; conduction loss and switching loss. These
losses are distributed between the upper and lower
MOSFETs according to duty factor (see the following
equations). The conduction losses are the main component
of power dissipation for the lower MOSFETs, Q2 and Q4 of
Figure 1. Only the upper MOSFETs, Q1 and Q3 have
significant switching losses, since the lower device turns on
and off into near zero voltage.
The equations assume linear voltage-current transitions and
do not model power loss due to the reverse-recovery of the
lower MOSFETs body diode. The reverse-recovery loss can
be a significant portion of the upper MOSFETs. The gate-
charge losses are dissipated by the Driver IC and don't heat
the MOSFETs. However, large gate-charge increases the
switching time, t_SW which increases the upper MOSFET
switching losses. Ensure that both MOSFETs are within their
maximum junction temperature at high ambient temperature
by calculating the temperature rise according to package
thermal-resistance specifications. A separate heatsink may
be necessary depending upon MOSFET power, package
type, ambient temperature and air flow.
P_UPPER
=
I_--O-----²_----×_-----r_{--D----S----(--O-----N----)---}×_-----V_{----O-----U----T-- + -}I_-O------×_-----V_{----I--N-----}×_-----t_{--S----W------}×_-----F_{----S----W---}
V_IN
2
P_LOWER
=
I_--O-----²_----×_-----r_{--D----S----(--O-----N----)---}×_-----(_---V_{----I--N-----–----}V_{-----O----U----T----})
V_IN
A diode, anode to ground, may be placed across Q2 and Q4
of Figure 1. These diodes function as a clamp that catches
the negative inductor swing during the dead time between
the turn off of the lower MOSFETs and the turn on of the
upper MOSFETs. The diodes must be a Schottky type to
prevent the lossy parasitic MOSFET body diode from
conducting. It is usually acceptable to omit the diodes and let
the body diodes of the lower MOSFETs clamp the negative
inductor swing, but efficiency could drop one or two percent
as a result. The diode's rated reverse breakdown voltage
must be greater than the maximum input voltage.
14