7
FN9004.2
November 12, 2004
Using the ISL6121EVAL1 Platform
General and Biasing Information
The ISL6121EVAL1 platform, Figure 17, allows evaluation of
the ISL6121 power supply control IC and comparison
against a suitably sized (1.85A hold current rating) PPTC
component. The evaluation platform is biased and monitored
through numerous test points (TP#). See Table 1 for test
point assignments and descriptions.
Upon proper bias the PPTC, F1, has a nominal 1.5A load
current passing through it which is the below the hold current
rating for that particular device. Removal of the PPTC is
necessary to isolate the ISL6121 as the PPTC load current
is common to the ISL6121EVAL1 bias connections.
By enabling the ISL6121H switches by signaling TP4 high
(> 2.4V) the IC is also loaded with a nominal 1.5A current.
Provided test points enable the evaluation of voltage loss
across the PPTC (TP9 - TP10) and likewise across the
ISL6121 enabled switch (TP9 - TP6). Expect to see 10% -
30% greater voltage loss across the PPTC than the
ISL6121.
An Overcurrent (OC) condition can be invoked on both the
ISL6121 and the PPTC by driving TP11 to +6V, causing
SW1 to close and a nominal 3A load is imposed on each.
This represents a current overload to the ISL6121 and is
thus quickly current regulated to the 2A limit. If the OC
duration extends beyond the nominal 12ms of the internal
ISL6121 timer then the output is latched off and the fault
output is asserted by being pulled low, turning on the
FAULT_ LED.
The primary function of any OC protection device is to
quickly isolate the voltage bus from a faulty load. Unlike the
PPTC and some other vendor available IC products, the
ISL6121 internal timer that starts upon OC detection
provides consistent protection that is independent of
temperature. Figures 14 through 16 illustrate the
comparative efficiency and effectiveness of the ISL6121 vs
the PPTC in protecting and isolating a faulty load capable
from drooping the system bus in that system.
TABLE 1. ISL6121EVAL1 TEST POINT ASSIGNMENTS
TP # DESCRIPTION
TP1 Eval Board and IC GND
TP2 Eval Board +5V Bias
TP4 Enable
TP5 Fault
TP6 VOUT
TP9 IC VIN Pin
TP10 PPTC Load Side
TP11 Invoke Over Current
FIGURE 14. ISL6121 vs PPTC INTO 3.3Ω LOAD FIGURE 15. ISL6121 vs PPTC INTO 1.6Ω LOAD
VIN = 5.07V
ISL6121 VOUT = 5V
PPTC VOUT = 4.96V
VOUT (100mV/DIV.)
PPTC VOUT
ISL6121 VOUT
12ms OC LATCH-OFF TIME
VOUT (1V/DIV.) TIME (2ms/DIV.)
ISL6121
