ISL6173 Intersil Corporation, ISL6173 Datasheet - Page 13

• Manufacturer Part Number: ISL6173
• Description: Dual Low Voltage Hot Swap Controller
• Manufacturer: Intersil Corporation

Applications Information
Selection of External Components
The typical application circuit of Figure 2 has been used for
this section, which provides guidelines to select the external
component values.
MOSFET (Q1)
This component should be selected on the basis of its
r
voltage) and the effective input gate capacitance (Ciss). One
needs to ensure that the combined voltage drop across the
Rsense and r
(including transients) will still keep the output voltage above
the minimum required level. Power dissipation in the device
under short circuit condition should also be an important
consideration especially in auto-retry mode (RTR/LTCH pin
pulled low). Using ISL6173 in latched off mode results in
lower power dissipation in the MOSFET.
Ciss of the MOSFET influences the overcurrent response
time. It is recommended that a MOSFET with Ciss of less
than 10nF be chosen. Ciss will also have an impact on the
SS cap value selection as seen later.
Current Sense Resistor (R
The voltage drop across this resistor, which represents the
load current (Io), is compared against the set threshold of
the current regulation amplifier. The value of this resistor is
determined by how much combined voltage drop is tolerable
between the source and the load. It is recommended that at
least 20mV drop be allowed across this resistor at max load
current. This resistor is expected to carry maximum full load
current indefinitely. Hence, the power rating of this resistor
must be greater than I
This resistor is typically a low value resistor and hence the
voltage signal appearing across it is also small. In order to
maintain high current sense accuracy, current sense trace
routing is critical. It is recommended that either a four wire
resistor or the following routing method be used:
DS(ON)
FIGURE 17. RECOMMENDED CURRENT SENSE RESISTOR
specification at the expected Vgs (gate to source
DS(ON)
PCB LAYOUT
LOAD CURRENT CARRYING
at the desired maximum current
O(MAX)
CURRENT
TRACES
TRACES
SENSE
13
2
*R
SNS
SNS
)
.
R
SNS
ISL6173
Current Set Resistor (R
This resistor directly sets the threshold for the current
regulation amplifier and indirectly sets the same for the OC
and WOC comparators in conjunction with R
R
calculate R
Reference Current Set Resistor (R
This resistor sets up the current in the internal current
source, I
voltage at the OCREF pin is the same as the internal
bandgap reference. The current (I
resistor is simply:
I
This current, I
internal current source as shown in Figure 2, because of the
4:1 current mirror. This equates to the resistor value of
14.7K.
Selection of Rs1 and Rs2
These resistors set the UV detect point. The UV comparator
detects the undervoltage condition when it sees the voltage
at UV pin drop below 0.633V. The resistor divider values
should be selected accordingly.
Charge Pump Capacitor Selection (C
C
charge pump, which operates at 450kHz set internally. The
output resistance of the charge pump, which affects the
regulation, is dependent on the C
charge-pump switch resistance, and the frequency and ESR
of the smoothing cap, C
It is recommended that C
(minimum) to 0.1µF (maximum) range. Only ceramic
capacitors are recommended. Use 0.1µF cap if CPVDD
output is expected to power an external circuit, in which case
the current draw from CPVDD must be kept below 10mA.
C
may be used if low ripple performance is desired.
Time-out Capacitor Selection (C
This capacitor controls the current regulation time-out
period. As shown in Figure 2, when the voltage across this
capacitor exceeds 1.178V, the time-out comparator detects it
and pulls down the gate voltage thus shutting down the
channel. An internal 10µA current source charges this
capacitor. Hence, the value of this capacitor is determined by
the following equation:
C
Where,
T
REF
OUT
SNS
P
V
T
is the “flying cap” and C
should at least be 0.47µF (ceramic only). Higher values
= (10µA * T
= 1.178/R
= Desired time-out period.
has been selected, use Equation 1 (on page 9) to
REF
SET
/4, shown in Figure 2 for the comparators. The
REF
. Use 20µA for I
OUT
REF
, should be set at 80µA to force 20µA in the
)/1.178
V
.
P
be kept within 0.022µF
V
SET)
is the smoothing cap of the
SET
P
REF
in a typical application.
value and its ESR,
) flowing through this
T
)
REF
P
SNS
)
and C
. Once
January 3, 2006
V
FN9186.3
)