MAX8869 MAXIM [Maxim Integrated Products], MAX8869 Datasheet - Page 8

• Manufacturer Part Number: MAX8869
• Description: 1A, Microcap, Low-Dropout, Linear Regulator Foldback Output Current Limit
• Manufacturer: MAXIM [Maxim Integrated Products]

below its nominal output voltage. RST remains low for
3ms after OUT has returned to its nominal value. A
100kΩ pullup resistor from RST to a suitable logic sup-
ply voltage (typically OUT) provides a logic control sig-
nal. RST can be used as a power-on-reset signal to a
microcontroller (µC) or can drive an external LED for
indicating a power failure. RST is low during shutdown.
RST remains valid for an input voltage as low as 1V.
As shown in Figure 4, a capacitor on SS allows a grad-
ual buildup of the MAX8869 output, reducing the initial
in-rush current peaks at startup. When SHDN is driven
low, the soft-start capacitor is discharged. When SHDN
is driven high or power is applied to the device, a con-
stant 6µA current charges the soft-start capacitor. The
resulting linear ramp voltage on SS increases the cur-
rent-limit comparator threshold, limiting the P-channel
gate drive (see Soft-Start Capacitor Selection ). Leave
SS unconnected to disable the soft-start feature.
The MAX8869 features a 4A current limit when the out-
put voltage is in regulation. When the output voltage
drops by 8% below its nominal, the current limit folds
back to 1.7A. The output can be shorted to ground for
an indefinite period of time without damaging the
device. Avoid continuous output current of more than
1A to prevent damage.
Both the in-regulation and short-circuit current limits can
be reduced from their nominal values by reducing the
voltage at the soft-start input (SS) below 1.25V. The cur-
rent limits scale proportionately with the voltage by I
= I
nominal 6µA current, the current limit can be set by
1A, Microcap, Low-Dropout,
Linear Regulator
Figure 3. Typical Operating Circuit with Adjustable Output
Voltage
8
V
LIM_NOM
IN
_______________________________________________________________________________________
1μF
ON
C
IN

OFF
(V
SS
/ 1.25). Since the SS input sources a
IN
IN
IN
IN
SHDN
SS
GND
OUT
OUT
OUT
OUT
RST
SET
Current Limiting
R1
R2
MAX8869
Soft-Start
C
1μF
OUT
RESET
OUTPUT
V
OUT
LIM
tying a resistor (R
= I
R
a capacitor (C
now ramps up asymptotically to the reduced current
limit rather than the nominal one, increasing the soft-
start time. The time required for the current limit to reach
90% of its steady-state value is given by t
R
Thermal overload protection limits total power dissipa-
tion in the MAX8869. When the junction temperature
exceeds T
pass transistor, allowing the IC to cool. The thermal sen-
sor turns the pass transistor on once the IC’s junction
temperature drops approximately 20°C. Continuous
short-circuit conditions will consequently result in a
pulsed output.
Thermal overload protection is designed to safeguard
the MAX8869 in the event of fault conditions. For con-
tinuous operation, do not exceed the absolute maxi-
mum junction temperature rating of T
Maximum power dissipation of the MAX8869 depends
on the thermal resistance of the case and circuit board,
the temperature difference between the die junction
and ambient air, and the rate of air flow. The power dis-
sipation across the device is P = I
resulting maximum power dissipation is:
where (T
between the maximum allowed die junction (150°C)
and the surrounding air; θ
Figure 4. Typical Operating Circuit with Soft-Start and Current-
Limit Reduction
SS
SS
V
LIM_NOM
IN
Operating Region and Power Dissipation
C
in place, soft-start can still be achieved by placing
SS
C
1μF
.
ON
SS
C
IN
P
J(MAX)
J
MAX

= +170°C, the thermal sensor turns off the
R
OFF
SS
SS
(I
= [(T
SS
) in parallel with R
SS
- T
Thermal Overload Protection

IN
IN
IN
IN
SHDN
SS
) between SS and GND, so that I
J(MAX)
A
R
) is the temperature difference
SS
JC
/ 1.25) where I
- T
GND
OUT
OUT
OUT
OUT
RST
SET
(junction
A
) / (θ
OUT
SS
JC
. The output current
J
= +150°C.
(V
+ θ
SS
IN
CA
MAX8869
- V
= 6µA . With
)]
C
1μF
OUT
OUT
SS
RESET
OUTPUT
= 2.3
). The
V
OUT
LIM