ISL6558EVAL1Z Intersil, ISL6558EVAL1Z Datasheet - Page 11

• Manufacturer Part Number: ISL6558EVAL1Z
• Description: EVAL BOARD 1 FOR ISL6558
• Manufacturer: Intersil

Specifications of ISL6558EVAL1Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OUTPUT SOFT-START AND TURN-OFF
With the input voltage (VCC12) staying within
operational limits, when the control voltage VCC5
reaches above the POR rising threshold (4.38V) of the
ISL6558, the FS/EN pin is released from ground and the
output begins a monotonic rise comprised of 2048 digital
steps, as shown in Figure 13. At the end of the soft-start
interval, the PGOOD signal transitions to indicate the
output voltage is within specification.
As shown in Figure 14, the converter is disabled when
the control voltage (VCC5) is pulled below the POR falling
threshold (3.88V) of the ISL6558. The PGOOD signal is
pulled low indicating the output voltage is out of
regulation. The ISL6612A enters Tri-State® and holds
both upper and lower drive signals low. The L-C resonant
tank is broken and cannot cause negative ringing at the
output since the lower FET is turned off, blocking any
negative current during the PWM Tri-State® period.
When the PWM input signal moves outside the shutdown
window, the ISL6612A is re-initialized; i.e., the lower
drive stays high and the upper drive stays low.
90
85
80
75
70
65
60
55
50
FIGURE 13. SOFT-START WAVEFORMS
0
10 20
FIGURE 12. EFFICIENCY
30
40
PGOOD
11
Iout (A)
Lo = 400nH
50 60
V
CONTROL
V
70
Lo = 600nH
O
(VCC5)
80 90 100
Application Note 1029
TRANSIENT RESPONSES
A transient load generator is populated on the board to
allow users to test high-speed load transients of the
converter. Current setting of the generator provides
about 120A load step with 100A/µs on the rising edge
and 140A/µs on the falling edge with output droop
configured. The input current rises/falls at a speed
limited by the input inductor and input capacitors during
step-up/step-down transients.
The transient performance for two different values of
output inductors have been compared and summarized
in Table 5. It shows that the lower the output inductance
is, the better the transient performance is, but it comes
at the expense of lower efficiencies, as illustrated in the
section entitled EFFICIENCY. In addition, the asymmetric
transient appears in the converter with higher output
inductance (Lo = 650nH), as can be easily seen in
Figures 19 and 22. This is because 650nH is much
greater than its critical inductance [6] (around 400nH or
smaller). Without the output droop configured, the
output has a much higher peak-to-peak voltage spike,
and it requires much more output capacitors to achieve
the same transient performance as that with droop
setting.
INDUCTANCE
VALUE
(nH)
400
400
400
400
650
650
650
650
FIGURE 14. TURN-OFF WAVEFORMS
TABLE 5. TRANSIENT RESPONSE
V
CONTROL
STEP-DOWN
Step-down
Step-down
STEP-UP/
Step-up
Step-up
V
O
(VCC5)
PGOOD
(mV)
Vpp
116
100
109
166
133
111
119
178
TRI-STATE®
IS REMOVED
DROOP FIGURE
x
x
x
x
x
x
July 31, 2009
AN1029.3
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