Datasheet

LM2585
www.ti.com SNVS120F –APRIL 2000–REVISED APRIL 2013
STABILITY
All current-mode controlled regulators can suffer from an instability, known as subharmonic oscillation, if they
operate with a duty cycle above 50%. To eliminate subharmonic oscillations, a minimum value of inductance is
required to ensure stability for all boost and flyback regulators. The minimum inductance is given by:
where
• V
SAT
is the switch saturation voltage and can be found in the Characteristic Curves. (6)
Figure 55. Circuit Board Layout
CIRCUIT LAYOUT GUIDELINES
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring
inductance generate voltage transients which can cause problems. For minimal inductance and ground loops,
keep the length of the leads and traces as short as possible. Use single point grounding or ground plane
construction for best results. Separate the signal grounds from the power grounds (as indicated in Figure 55).
When using the Adjustable version, physically locate the programming resistors as near the regulator IC as
possible, to keep the sensitive feedback wiring short.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2585 junction temperature within the allowed operating
range. For each application, to determine whether or not a heat sink will be required, the following must be
identified:
1) Maximum ambient temperature (in the application).
2) Maximum regulator power dissipation (in the application).
3) Maximum allowed junction temperature (125°C for the LM2585). For a safe, conservative design, a
temperature approximately 15°C cooler than the maximum junction temperature should be selected (110°C).
4) LM2585 package thermal resistances θ
JA
and θ
JC
(given in the Electrical Characteristics).
Total power dissipated (P
D
) by the LM2585 can be estimated as follows:
where
• V
IN
is the minimum input voltage
• V
OUT
is the output voltage
Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 23
Product Folder Links: LM2585