ISL6173 Datasheet, PDF (9/20 Pages) Intersil Corporation – Dual Low Voltage Hot Swap Controller

ISL6173
Detailed Description of Operation
ISL6173 targets dual voltage hot-swap applications with a
bias of 2.1V to 3.6VDC and the voltages being controlled
down to 0.7VDC. The IC’s main function is to limit and
regulate the inrush current into the loads. This is achieved by
enhancing an external MOSFET in a controlled manner. In
order to fully enhance the MOSFET, the IC must provide
adequate gate to source voltage, which is typically 5V or
greater. Hence, the final steady-state voltage on Gate (GT)
pin must be 5V above the load voltage. Two internal charge-
pumps allow this to happen.
VIN
VO
Q
10V
24µA
SOFT-
START
AMPLIFIER
-
42µA
+
0
CPVDD
10µA ⁰
SS1
VIN
CPVDD
+
-
FIGURE 12. SOFT-START OPERATION
Controlled Soft-Start
The output voltages are monitored through the Vo pins and
slew up at a rate determined by the capacitors on the Soft-
start (SS) pin, as illustrated in Figure 12. 24µA of gate
charge current is available. The soft-start amplifier controls
the output voltage by robbing some of the gate charge
current thus slowing down the MOSFET enhancement.
When the load voltage reaches its set level, as sensed by its
respective UV pin through an external resistor divider, the
Power Good (PG) output goes active.
Current Monitoring and Protection
The IC monitors the load current (Io) by sensing the voltage-
drop across the low value current sense resistor (R_SNS),
which is connected in series with the MOSFET as shown in
the diagram on page 2, through Sense (SNS) and voltage
set (VS) pins. The latter is through a resistor, R_SET, as
shown. Two levels of overcurrent detection are available to
protect against all possible fault scenarios. These levels are:
1. Current Limit or Current Regulation (CR)
2. Way Overcurrent (WOC)
Each of these modes is described in detail as follows:
1. Current Limit or Current Regulation (CR) Mode: - When
the load current reaches the current regulation threshold, the
current amplifier loop closes and the circuit behaves like a
current source. The Current Limit Amplifier is a folded
cascode type with source follower output capable of pulling
down the gate very fast in response to fast overload
transients. The current regulation threshold is set by setting a
reference current, I_SET, through R_SET by selecting an
appropriate resistor between OCREF and GND, which sets
I_REF. The relationship between I_REF and I_SET is I_REF =
4*I_SET, where I_REF = Vocref/Rocref = 1.178/Rocref. I_REF
would typically be set at 80µA.
Selecting appropriate values for R_SET and R_SNS such that
when I_O = I_CR,
Io*R_SNS = I_SET*R_SET
(EQ. 1)
Vin
+
-
Vo
+
Iset ^Rset
-
Rsns
Q
CURRENT REGULATION
MODE:
Iset*Rset = Io*Rsns
ISL6173
3K
CURRENT 10V
LIMIT
AMPLIFIER
24µA
-
+
Iref
4
FIGURE 13. CURRENT REGULATION OPERATION
The operating mode is shown in Figure 13. When the circuit
enters this mode, the OC comparator detects it and sets off
the timer. CT begins to charge from an internal 10µA current
source. The amount of time it takes for this cap to charge to
1.178V sets up the current regulation duration. Upon
expiration of this time-out period, the MOSFET gate is pulled
down quickly by the current limit amplifier, unless the load
current level had already dropped back to a level below the
current regulation threshold level prior to that. In that case, the
current regulation mode is no longer active, the MOSFET is
allowed to fully enhance and the IC discharges the C_T Cap. If
RTR/LTCH pin is left open or pulled to BIAS, the output
remains latched off after the expiration of the time-out period
determined by C_T. If RTR/LTCH pin is pulled to GND, the IC
9
FN9186.2
February 15, 2005