LT1766 Datasheet, PDF (11/28 Pages) Linear Technology – Step-Down Switching Regulator

LT1766/LT1766-5
APPLICATIO S I FOR ATIO
materials fall somewhere in between. The following for-
mula assumes continuous mode of operation, but errs
only slightly on the high side for discontinuous mode, so
it can be used for all conditions.
( _{( )()( )( )( )} ) _IPEAK
=
I_OUT
+
(I_LP-P )
2
=
I_OUT
+
V_OUT
2
V_IN – V_OUT
V_IN f L
EMI
Decide if the design can tolerate an “open” core geometry
like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid to
prevent EMI problems. This is a tough decision because
the rods or barrels are temptingly cheap and small and
there are no helpful guidelines to calculate when the
magnetic field radiation will be a problem.
Additional Considerations
After making an initial choice, consider additional factors
such as core losses and second sourcing, etc. Use the
experts in Linear Technology’s Applications department if
you feel uncertain about the final choice. They have
experience with a wide range of inductor types and can tell
you about the latest developments in low profile, surface
mounting, etc.
Maximum Output Load Current
Maximum load current for a buck converter is limited by
the maximum switch current rating (I_P). The current rating
for the LT1766 is 1.5A. Unlike most current mode convert-
ers, the LT1766 maximum switch current limit does not
fall off at high duty cycles. Most current mode converters
suffer a drop off of peak switch current for duty cycles
above 50%. This is due to the effects of slope compensa-
tion required to prevent subharmonic oscillations in cur-
rent mode converters. (For detailed analysis, see Applica-
tion Note 19.)
The LT1766 is able to maintain peak switch current limit over
the full duty cycle range by using patented circuitry* to cancel
the effects of slope compensation on peak switch current
without affecting the frequency compensation it provides.
*Patent # 6, 498, 466
Maximum load current would be equal to maximum
switch current for an infinitely large inductor, but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current (I_LP-P). The follow-
ing formula assumes continuous mode operation, imply-
ing that the term on the right is less than one-half of I_P.
I_OUT(MAX) =
Continuous Mode
I_P
–
I_LP-P
2
=
I_P
−
(^V_OUT
+
V_{F )(}V_IN − V_OUT
2₍L₎₍f₎₍V_IN)
–
V_{F )}
For V_OUT = 5V, V_IN = 8V, V_F(D1) = 0.63V, f = 200kHz and
L = 20µH:
( )( ) ^IOUT(MAX)
=
1.5
−
2
(^{5 + 0.63})(^{8 − 5}
20 • 10^−6 200
– 0.63₎
• 10³ ₍8₎
= 1.5 − 0.21= 1.29A
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V_IN = 15V, duty cycle is 33% and for the same set of
conditions:
( )( ) ^IOUT(MAX)
=
1.5
−
2
(^{5 + 0.63})(^{15 − 5}
20 • 10^−6 200 •
– 0.63₎
10³ ₍15₎
= 1.5 − 0.44 = 1.06A
To calculate actual peak switch current with a given set of
conditions, use:
I_SW(PEAK)
=
I_OUT
+
I_LP-P
2
=
I_OUT
+
(V_OUT
+
( ^V_F ^{) V}_IN ^{− V}_OUT
2₍L₎₍f₎₍V_IN)
–
V_{F )}
Reduced Inductor Value and Discontinuous Mode
If the smallest inductor value is of most importance to a
converter design, in order to reduce inductor size/cost,
discontinuous mode may yield the smallest inductor solu-
tion. The maximum output load current in discontinuous
mode, however, must be calculated and is defined later in
this section.
1766fa
11