ISL6326CRZ-T Intersil, ISL6326CRZ-T Datasheet - Page 28

• Manufacturer Part Number: ISL6326CRZ-T
• Description: IC CTRLR PWM 4PHASE BUCK 40-QFN
• Manufacturer: Intersil

Specifications of ISL6326CRZ-T

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 275kHz
• Duty Cycle: 25%
• Voltage - Supply: 4.75 V ~ 5.25 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 40-VFQFN, 40-VFQFPN
• Frequency-max: 275kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

.
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
ΔV
Equation 39 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output voltage deviation than the leading edge. Equation 40
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
Switching Frequency Selection
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper-MOSFET loss calculation. These effects are
outlined in “MOSFETs” on page 24 and they establish the
upper limit for the switching frequency. The lower limit is
established by the requirement for fast transient response
and small output voltage ripple as outlined in “Output Filter
Design” on page 27. Choose the lowest switching frequency
that allows the regulator to meet the transient-response
requirements.
Input Capacitor Selection
The input capacitors are responsible for sourcing the AC
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the AC component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
L
L
L
≤
≤
≥
MAX
2NCV
-------------------- - ΔV
(
------------------------- - ΔV
1.25
(
(
ESR
ΔI
(
ΔI
. This places an upper limit on inductance.
)
) NC
)
2
O
2
)
⎛
⎝
----------------------------------------------------------- -
V
IN
f
S
MAX
V
–
MAX
IN
N V
V
–
P-P MAX
OUT
ΔI ESR
–
ΔI ESR
(
(
(
⎞ V
⎠
OUT
)
)
28
)
⎛
⎝
V
IN
–
V
O
⎞
⎠
(EQ. 38)
(EQ. 39)
(EQ. 40)
ISL6326
For a two-phase design, use Figure 18 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I
of the per-phase peak-to-peak inductor current (I
I
will minimize the total number of input capacitors required to
support the RMS current calculated. The voltage rating of
the capacitors should also be at least 1.25x greater than the
maximum input voltage.
O
FIGURE 18. NORMALIZED INPUT-CAPACITOR RMS CURRENT
FIGURE 19. NORMALIZED INPUT-CAPACITOR RMS CURRENT
. Select a bulk capacitor with a ripple current rating which
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
I
I
L(P-P)
L(P-P)
I
I
I
L(P-P)
L(P-P)
L(P-P)
= 0
= 0.25 I
vs DUTY CYCLE FOR 2-PHASE CONVERTER
vs DUTY CYCLE FOR 3-PHASE CONVERTER
= 0
= 0.5 I
= 0.75 I
0.2
0.2
O
O
O
DUTY CYCLE (V
DUTY CYCLE (V
0.4
0.4
I
I
L(P-P)
L(P-P)
= 0.5 I
= 0.75 I
0.6
0.6
O
O/
/V
V
IN
IN
O
)
)
O
O
), and the ratio
0.8
0.8
L(P-P)
May 5, 2008
FN9262.1
) to
1.0
1.0