Datasheet

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RMS
= I
OUTMAX
x
r =
V
OUT
V
IN
r
2
12
1 -
+
V
OUT
V
IN
¸
¸
¹
·
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§
x
(V
IN
- V
OUT
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OUTMAX
V
IN
x x x
LM3691
www.ti.com SNVS506I –MAY 2008–REVISED MAY 2013
THERMAL SHUTDOWN PROTECTION
The LM3691 has a thermal overload protection function that operates to protect itself from short-term misuse and
overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both
the PFET and the NFET are turned off. When the temperature drops below 130°C, normal operation resumes.
Prolonged operation in thermal overload conditions may damage the device and is considered bad practice.
OVER-TEMPERATURE MAXIMUM LOAD RECOMMENDATIONS
VIN Maximum Load
2.5V to 5.5V 1000 mA
2.3V to 2.5V 650 mA
APPLICATION INFORMATION
INDUCTOR SELECTION
DC bias current characteristics of inductors must be considered. Different manufacturers follow different
saturation current rating specifications, so attention must be given to details. DC bias curves should be requested
from them as part of the inductor selection process.
Minimum value of inductance to specify good performance is 0.5 µH at 1.5A (I
LIM
typ.) bias current over
the ambient temp range. The inductor’s DC resistance should be less than 0.1Ω for good efficiency at high
current condition. The inductor AC loss (resistance) also affects conversion efficiency. Higher Q factor at
switching frequency usually gives better efficiency at light load to middle load.
Table 1 lists suggested inductors and suppliers.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF, 6.3V/10V is sufficient for most applications. Place the input capacitor as
close as possible to the V
IN
pin and GND pin of the device. A larger value or higher voltage rating may be used
to improve input voltage filtering. Use X7R, X5R or B types; do not use Y5V or F. DC bias characteristics of
ceramic capacitors must be considered when selecting case sizes like 0402. Minimum input capacitance to
ensure good performance is 2.2 µF at maximum input voltage DC bias including tolerances and over
ambient temp range.
The input filter capacitor supplies current to the PFET (high-side) switch in the first half of each cycle and
reduces voltage ripple imposed on the input power source. A ceramic capacitor's low ESR provides the best
noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with
sufficient ripple current rating. The input current ripple can be calculated as:
(1)
OUTPUT CAPACITOR SELECTION
Use a 4.7μF, 6.3V ceramic capacitor, X7R, X5R or B types; do not use Y5V or F. DC bias voltage characteristics
of ceramic capacitors must be considered. DC bias characteristics vary from manufacturer to manufacturer, and
DC bias curves should be requested from them as part of the capacitor selection process. The output filter
capacitor smooths out current flow from the inductor to the load, helps maintain a steady output voltage during
transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient
capacitance and sufficiently low ESR to perform these functions. Minimum output capacitance to specify
good performance is 2.2 µF at the output voltage DC bias including tolerances and over ambient temp
range.
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