Datasheet

2
L1 OUT L
P I R 1.1= ´ ´
LM5006
www.ti.com SNVS646B –FEBRUARY 2011–REVISED MARCH 2013
current switches between these two loops with each transition from on-time to off-time and back to on-time, it is
imperative that the ground end of C1 have a short and direct connection to D1’s anode (or Q1’s source), without
going through vias or a lengthy route. The power dissipation in the LM5006 can be approximated by determining
the total conversion loss (P
IN
– P
OUT
), and then subtracting the power losses in D1 (or Q1), and in the inductor.
The power loss in the diode is approximately:
P
D1
= I
OUT
x V
F
x (1–D)
where
• V
F
is the diode’s forward voltage drop
• D is the on-time duty cycle (24)
The average power dissipation in the synchronous rectifier (Q1) is calculated from:
P
Q1
= I
OUT
2
x R
DS (on)
x (1–D)
where
• R
DS(on)
is the device’s on-resistance
• D is the on-time duty cycle (25)
The power loss in the inductor is approximately:
where
• R
L
is the inductor DC resistance
• the 1.1 factor is an approximation for the AC losses (26)
If it is expected that the internal dissipation of the LM5006 will produce excessive junction temperatures during
normal operation, good use of the PC board’s ground plane can help to dissipate heat. Additionally the use of
wide PC board traces, where possible, can help conduct heat away from the IC. Judicious positioning of the PC
board within the end product, along with the use of any available air flow (forced or natural convection) can help
reduce the junction temperature.
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