ADM869L AD [Analog Devices], ADM869L Datasheet - Page 7

• Manufacturer Part Number: ADM869L
• Description: 2 A, High-Side P-Channel Switch with Current Limit and Thermal Shutdown
• Manufacturer: AD [Analog Devices]

FUNCTIONAL DESCRIPTION
The ADM869L comprises a high-current P-channel switch con-
trolled by an active-low logic input ON (Pin 7). When ON is
low, the internal circuitry of the ADM869 is powered up and the
output of the current-limit amplifier is low, providing gate drive
to the switching FET. When ON is high, the internal circuitry is
powered down and the current consumption is typically 100 nA.
It should be noted that the ADM869L is not a bidirectional
switch, so V
CURRENT LIMIT
When the switch is turned on, a smaller mirror switch passes a
proportionate current equal to I
maintains this relationship by keeping the drain of the mirror
FET at the same voltage as the main FET, and drives the mirror
current through the current-limit resistor RSET, which is
connected between the noninverting input of the current limit
amplifier and ground. An on-chip bandgap reference of 1.24 V
is connected to the inverting input of the current-limit amplifier.
When the load current exceeds the preset limit, the voltage
across R
current-limit amplifier rises, reducing the gate drive to the FETs.
If, for example, a 1 A current limit is required, R
1.047 mA = 1.184 kΩ. Note that I
current so please refer to TPC 6.
THERMAL SHUTDOWN
The thermal shutdown operates when the die temperature
exceeds 135°C, turning off the switch. The thermal shutdown
circuit has built-in hysteresis of 10°C, so the switch will not turn
on again until the die temperature falls to 125°C. If the fault
condition is not removed, the switch will pulse on and off as the
temperature cycles between these limits.
FAULT OUTPUT
If either the current limit or the thermal shutdown is activated,
FAULT will pull low. This is an open-drain output and requires
a pull-up resistor of between 10 kΩ and 100 kΩ. Several FAULT
outputs may be wire-OR’d to form a common interrupt line, as
shown in Figure 2 or FAULT may be wire-OR’d to an existing
interrupt line that has a resistive pull-up.
SET
IN
exceeds 1.24 V, and the output voltage of the
must always be higher than V
ADM869L
SET
OUT
1.24V
I
LIMIT
OUT
955
/955. The mirror amplifier
/I
SET
R
SET
varies depending on
OUT
.
SET
= 1.24 V/
During startup, the FAULT output goes low for the turn-on time
plus 50 µs.
APPLICATIONS INFORMATION
INPUT FILTERING
To prevent the input voltage being pulled below the minimum
operating voltage under transient short-circuit conditions, before
the current limit has had time to operate, a reservoir capacitor
should be connected from IN to GND. This does not need to be
large, but should have a low ESR. A 22 µF ceramic type is
suitable. Larger values will reduce the voltage drop still further.
OUTPUT CAPACITANCE
A 0.1 µF capacitor should be connected between OUT and
GND to prevent the back e.m.f. of parasitic inductance from
pulling OUT below ground during turn-off. For Universal Serial
Bus (USB) applications, C
causes the output rise and fall times to be longer, as shown in
the typical operating characteristics, but does not affect the
turn-off time of the ADM869L itself.
LAYOUT CONSIDERATIONS
Printed circuit board tracks to and from the ADM869L should be
as thick and as short as possible to minimize parasitic inductance
and take full advantage of the fast response time of the switch. It is
recommended that all input lines be connected together, close to
the device. This ensures equal current distribution in all legs. If
this is not possible, then all traces should be of equal width and
length. The same rules apply for all output lines. Input and output
capacitors should be placed as close to the device as possible (less
than 5 mm).
THERMAL CONSIDERATIONS
Under normal operating conditions, the worst-case power dissi-
pation will be 518 mW with the highest specified on resistance
and 3 V supply (W = 2.4 A × 0.9 V). The package is capable of
handling and dissipating this power, but heat dissipation can
further be improved by providing a large area of copper in con-
tact with the device pins, particularly IN and OUT.
ADM869L
FAULT
ADM869L
FAULT
OUT
should be at least 120 µF. This
ADM869L
FAULT
FAULT
ADM869L
V
PULLUP
R
PULLUP
INT