ISL78208_14 Datasheet, PDF (16/24 Pages) Intersil Corporation – Wide VIN Dual Standard Buck Regulator with 3A/3A Continuous Output Current

ISL78208
shorted to GND with VOUT1 in regulation, it will enter overcurrent
hiccup mode with a very short hiccup waiting time. The reason is
that VOUT1 is still in regulation and can pull-up SS2 very quickly
via the resistor added between VOUT1 and SS2.
Figure 42 illustrates output sequencing. When EN1 is high and
EN2 is floating, OUT1 comes up first and OUT2 won't start until
OUT1 > 90% of its regulation point. If EN1 is floating and EN2 is
high, OUT2 comes up first and OUT1 won't start until OUT2 > 90%
of its regulation point. If EN1 = EN2 = high, OUT1 and OUT2 come
up at the same time. Please refer to Table 1 for conditions
related to Figure 42 (Output Sequencing).
TABLE 1. OUTPUT SEQUENCING
EN1
EN2
V_OUT1
V_OUT2
NOTE
High Floating
First
After V_OUT1 > 90%
Floating High After V_OUT2 > 90%
First
High High
Same time
as V_OUT2
Same time
as V_OUT1
Floating Floating
Not
Allowed
VCC
C2
10nF
SS1
V_OUT1
SS2
ISL78208
V_OUT2
5.0V
C₃
3.3V
C₄
FIGURE 39. INDEPENDENT START-UP
VCC
VCC
SS1
V_OUT1
SS2
ISL78208
V_OUT2
5.0V
C₃
3.3V
C₄
FIGURE 40. RATIOMETRIC START-UP
VCC
SS1
V_OUT1
5.0V
C₃
SS2
ISL78208
V_OUT2
3.3V
C₄
R₂
8.06k
R₁
25.5k
FIGURE 41. ABSOLUTE START-UP
VCC
VCC
SS1
V_OUT1
SS2
EN1
ISL78208
V_OUT2
EN2
5.0V
C₃
3.3V
C₄
FIGURE 42. OUTPUT SEQUENCING
Protection Features
The ISL78208 limits the current in all on-chip power devices.
Overcurrent protection limits the current on the two buck
regulators and internal LDO for V_CC.
Buck Regulator Overcurrent Protection
During the PWM on-time, the current through the internal
switching MOSFET is sampled and scaled through an internal
pilot device. The sampled current is compared to a nominal 5A
overcurrent limit. If the sampled current exceeds the overcurrent
limit reference level, an internal overcurrent fault counter is set
to 1 and an internal flag is set. The internal power MOSFET is
immediately turned off and will not be turned on again until the
next switching cycle.
The protection circuitry continues to monitor the current and
turns off the internal MOSFET as described. If the overcurrent
condition persists for 17 sequential clock cycles, the overcurrent
fault counter overflows indicating an overcurrent fault condition
exists. The regulator is shut down and power-good goes low.
The buck controller attempts to recover from the overcurrent
condition after waiting 8 soft-start cycles. The internal
overcurrent flag and counter are reset. A normal soft-start cycle
is attempted and normal operation continues if the fault
condition has cleared. If the overcurrent fault counter overflows
during soft-start, the converter shuts down and this hiccup mode
operation repeats.
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FN8354.1
July 29, 2014