Datasheet

V
OUT
= V
FB
x
(R
FB1
+ R
FB2
)
R
FB1
LM3150
www.ti.com SNVS561D –SEPTEMBER 2008–REVISED MARCH 2011
Enable/Shutdown
The EN pin can be activated by either leaving the pin floating due to an internal pull up resistor to VIN or by
applying a logic high signal to the EN pin of 1.26V or greater. The LM3150 can be remotely shut down by taking
the EN pin below 1.02V. Low quiescent shutdown is achieved when V
EN
is less than 0.4V. During low quiescent
shutdown the internal bias circuitry is turned off.
The LM3150 has certain fault conditions that can trigger shutdown, such as over-voltage protection, current limit,
under-voltage lockout, or thermal shutdown. During shutdown, the soft-start capacitor is discharged. Once the
fault condition is removed, the soft-start capacitor begins charging, allowing the part to start-up in a controlled
fashion. In conditions where there may be an open drain connection to the EN pin, it may be necessary to add a
1 nF bypass capacitor to this pin. This will help decouple noise from the EN pin and prevent false disabling.
Thermal Protection
The LM3150 should be operated such that the junction temperature does not exceed the maximum operating
junction temperature. An internal thermal shutdown circuit, which activates at 165°C (typical), takes the controller
to a low-power reset state by disabling the buck switch and the on-timer, and grounding the SS pin. This feature
helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back
below 150°C the SS pin is released and device operation resumes.
Design Guide
The design guide provides the equations required to design with the LM3150 SIMPLE SWITCHER
®
Controller.
WEBENCH
®
design tool can be used with or in place of this section for a more complete and simplified design
process.
1. Define Power Supply Operating Conditions
a. Required Output Voltage
b. Maximum and Minimum DC Input Voltage
c. Maximum Expected Load Current during Normal Operation
d. Soft-Start Time
2. Set Output Voltage With Feedback Resistors
where
• R
FB1
is the bottom resistor
• R
FB2
is the top resistor (13)
3. Determine R
ON
and f
s
The available frequency range for a given input voltage range, is determined by the duty-cycle, D = V
OUT
/V
IN
, and
the minimum t
ON
and t
OFF
times as specified in the Electrical Characteristics table. The maximum frequency is
thus, f
smax
= D
min
/t
ON-MIN
. Where D
min
=V
OUT
/V
IN-MAX
, is the minimum duty-cycle. The off-time will need to be less
than the minimum off-time t
OFF
as specified in the Electrical Characteristics table plus any turn off and turn on
delays of the MOSFETs which can easily add another 200 ns. The minimum off-time will occur at maximum duty
cycle D
max
and will determine if the frequency chosen will allow for the minimum desired input voltage. The
requirement for minimum off-time is t
OFF
= (1–D
max
)/f
s
≥ (t
OFF-MIN
+ 200 ns). If t
OFF
does not meet this requirement
it will be necessary to choose a smaller switching frequency f
S
.
Choose R
ON
so that the switching frequency at your typical input voltage matches your f
S
chosen above using
the following formula:
R
ON
= [(V
OUT
x V
IN
) - V
OUT
] / (V
IN
x K x f
S
) + R
OND
(14)
R
OND
= - [(V
IN
- 1) x (V
IN
x 16.5 + 100)] - 1000 (15)
Use Figure 13 to determine if the calculated R
ON
will allow for the minimum desired input voltage. If the minimum
desired input voltage is not met, recalculate R
ON
for a lower switching frequency.
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