ISL6115_07 Datasheet, PDF (6/12 Pages) Intersil Corporation – Power Distribution Controllers

ISL6115, ISL6116, ISL6117, ISL6120
Application Considerations
During the soft-start and the time-out delay duration with the
IC in its current limit mode, the V_GS of the external N-Channel
MOSFET is reduced driving the MOSFET switch into a (linear
region) high r_DS(ON) state. Strike a balance between the CR
limit and the timing requirements to avoid periods when the
external N-Channel MOSFETs may be damaged or destroyed
due to excessive internal power dissipation. Refer to the
MOSFET SOA information in the manufacturer’s data sheet.
When driving particularly large capacitive loads a longer soft-
start time to prevent current regulation upon charging and a
short CR time may offer the best application solution relative
to reliability and FET MTF.
Physical layout of R_SENSE resistor is critical to avoid the
possibility of false overcurrent occurrences. Ideally, trace
routing between the R_SENSE resistors and the IC is as direct
and as short as possible with zero current in the sense lines
(See Figure 1).
R_CL
1.58kΩ
1W
LOAD
0.001µF
2kΩ
0.005
1%
1.47kΩ
1%
0.01µF
ISL6116
12V
DD1
0.047µF
NC
PWRON
V_{BUS -48V}
FIGURE 2.
CORRECT
INCORRECT
TO ISEN AND
R_ISET
CURRENT
SENSE RESISTOR
Biasing the ISL6116
Table 3 gives typical component values for biasing the
ISL6116 in a ±48V application. The formulas and
calculations deriving these values are also shown below.
TABLE 3. TYPICAL VALUES FOR A -48V HOT SWAP
APPLICATION
SYMBOL
PARAMETER
R_CL
DD1
1.58kΩ, 1W
12V Zener Diode, 50mA Reverse Current
FIGURE 1. SENSE RESISTOR PCB LAYOUT
Using the ISL6116 as a -48V Low Side Hot
Swap Power Controller
To supply the required V_DD, it is necessary to maintain the
chip supply 10 to 16V above the -48V bus. This may be
accomplished with a suitable regulator between the voltage
rail and pin 5 (VDD). By using a regulator, the designer may
ignore the bus voltage variations. However, a low-cost
alternative is to use a Zener diode (See Figure 2 for typical
5A load control); this option is detailed below.
Note that in this configuration the PGOOD feature (pin 7) is
not operational as the I_SEN pin voltage is always < UV
threshold.
See Figures 17 to 20 for waveforms relevant to -48V and
other high voltage applications.
When using the ISL6116 to control -48V, a Zener diode may
be used to provide the +12V bias to the chip. If a Zener is
used then a current limit resistor should also be used.
Several items must be taken into account when choosing
values for the current limit resistor (R_CL) and Zener Diode
(DD1):
• The variation of the V_BUS (in this case, -48V nominal)
• The chip supply current needs for all functional conditions
• The power rating of R_CL.
• The current rating of DD1
Formulas
1. Sizing R_CL:
R_CL = (V_BUS,MIN - 12)/I_CHIP
2. Power Rating of R_CL:
P_RCL = I_C(V_BUS,MAX - 12)
3. DD1 Current Rating:
I_DD1 = (V_BUS,MAX - 12)/R_CL
6
February 6, 2007