FAN1539 Fairchild Semiconductor, FAN1539 Datasheet - Page 11

• Manufacturer Part Number: FAN1539
• Description: 1A/1.3A/ LDO With Low Quiescent Current
• Manufacturer: Fairchild Semiconductor

Applications Information
General Circuit Description
The FAN1539/FAN1540 is an advanced low dropout voltage
regulator, specially designed for applications in portable
computers, where high performance and low quiescent cur-
rent is required. The device has an internal trimmed bandgap
voltage reference and an internal output voltage sense
divider. These two signals form the input to the error ampli-
fier which regulates the output voltage.
The FAN1539/FAN1540 has a complete set of internal pro-
tection circuitry including thermal shutdown, short circuit
current limit and electrostatic discharge protection. Low
ESR ceramic capacitors are needed for input as well as out-
put pins to maintain the circuit stability.
Short Circuit Current Limit
The device has internal over-current limit and short circuit
protection. Under over-current conditions the device current
is determined by the current limit threshold. Once the device
is released from short circuit conditions, the normal level of
current limit is gradually re-established as the device output
voltage reaches normal levels. Special circuitry has been
added to ensure that recovery from short circuit current con-
ditions does not lead to excessive overshoot of the output
voltage -- a phenomenon often encountered in conventional
regulators.
Thermal Protection
The FAN1539/FAN1540 is designed to supply at least 1A/
1.3A output currents. Excessive output load at high input -
output voltage difference will cause the device temperature
to increase and exceed maximum ratings due to power dissi-
pation. During output overload conditions, when the die tem-
perature exceeds the shutdown limit temperature of 160°C,
an onboard thermal protection will disable the output until
the temperature drops approximately 15°C below the limit,
at which point the output is re-enabled.
Thermal Characteristics
The FAN1539/FAN1540 is designed to supply at least 1A/
1.3A at the specified output voltage with an operating die
(junction) temperature of up to 125°C. Once the power dissi-
pation and thermal resistance is known, the maximum junc-
tion temperature of the device can be calculated. While the
power dissipation is calculated from known electrical param-
eters, the actual thermal resistance depends on the thermal
characteristics of the chosen package and the surrounding
PC board copper to which it is mounted.
11
FAN1539/FAN1540
The power dissipation is equal to the product of the input-to-
output voltage differential and the output current plus the
ground current multiplied by the input voltage, or:
The ground pin current, I
provided in the “Electrical Characteristics” section.
The relationship describing the thermal behavior of the
package is:
where T
ture of the die, which is 150°C, and T
ing temperature.
board layout and can be empirically obtained. While the
(junction-to-case) of the 6-lead MLP package is specified at
8°C /W, the
substantially higher. This can be improved upon by provid-
ing a heat sink of surrounding copper ground on the PWB.
Depending on the size of the copper area, and the thickness
of the copper layer, the resulting
range. The addition of backside copper with through-holes,
stiffeners, and other enhancements can also aid in reducing
thermal resistance. Thermal simulations performed on a
thermally optimized board layout indicate that
as 20°C /W can be achieved. For example, the heat contrib-
uted by the dissipation of other devices located nearby must
be included in the design considerations.
Overload conditions also need to be considered. It is possible
for the device to enter a thermal cycling loop, in which the
circuit enters a shutdown condition, cools, re-enables, and
then again overheats and shuts down repeatedly due to a per-
sistent fault condition.
Capacitor ESR and Printed Circuit Board Layout
The FAN1539/FAN1540 has been optimized to accommo-
date low ESR bypass capacitors down to 0 m . For best
results it is important to place both input and output bypass
capacitors as near to the input and output pins as possible.
Use of X7R types such as Murata’s
GRM31CR70J106KA01B (10 µF) and
GRM43ER71A226KE01B (22 µF) or similar component
from TDK. The capacitors should connect directly to the
ground plane. Use of ground plane on the top and the bottom
side of the PCB is recommended. As many via as possible
should be used to minimize ground plane resistance.
J(max)
P
D
JA
=
is the maximum allowable junction tempera-
P
for a minimum PWB footprint will be in
D max
V
JA
IN
is dependent on the surrounding PC
–
=
V
GND
OUT



T
----------------------------- -
J max
can be found in the charts
I
OUT
JA
JA
PRODUCT SPECIFICATION
–
+
can vary over a wide
A
T
V
is the ambient operat-
A
IN



REV. 1.0.8 3/22/04
I
GND
JA
as low
JC