Datasheet

C
IN
=
I
OUT(max)
x t
ON(max)
'V
= 2.02 PF
1.5A x 672 ns
0.5V
=
Duty Cycle = D =
=
= 13.9%
V
OUT
V
IN
5V
36V
LM25011, LM25011-Q1
SNVS617G –APRIL 2009–REVISED FEBRUARY 2013 www.ti.com
R
S
: The minimum current limit threshold is calculated at maximum load current, using the minimum ripple current
calculated above. The current limit threshold is the lower peak of the inductor current waveform when in current
limit (see Figure 18).
I
LIM
= 1.5A – (0.2 A/2) = 1.4A (14)
Current limit detection occurs when the voltage across the sense resistor (R
S
) reaches the current limit threshold.
To allow for tolerances, the sense resistor value is calculated using the minimum threshold specification:
R
S
= 115 mV/1.4A = 82 mΩ (15)
The next smaller standard value, 80 mΩ, is selected. The next step is to ensure that sufficient ripple voltage
occurs across R
S
with this value sense resistor. As mentioned in the Ripple Requirements section, a minimum of
15 mVp-p voltage ripple is required across the R
S
sense resistor during the off-time to ensure the regulation
circuit operates properly. The ripple voltage is the product of the inductor ripple current amplitude and the sense
resistor value. In this case, the minimum ripple voltage calculates to:
V
RIPPLE
= ΔI x R
S
= 200 mA x 0.080Ω = 16 mV (16)
If the ripple voltage had calculated to less than 15 mVp-p the inductor value would have to be reduced to
increase the ripple current amplitude. This would have required a recalculation of I
LIM
and R
S
in the above
equations. Since the minimum requirement is satisfied in this case no change is necessary.
The nominal current limit threshold calculates to 1.63A. The minimum and maximum thresholds calculate to
1.44A and 1.83A respectively, using the minimum and maximum limits for the current limit threshold
specification. The load current is equal to the threshold current plus one half the ripple current. Under normal
load conditions, the maximum power dissipation in R
S
occurs at maximum load current, and at maximum input
voltage where the on-time duty cycle is minimum. In this design example, the minimum on-time duty cycle is:
(17)
At maximum load current, the power dissipation in R
S
is equal to:
P
(RS)
= (1.5A)
2
x 0.080Ω x (1 – 0.139) = 155 mW (18)
When in current limit the maximum power dissipation in R
S
calculates to
P
(RS)
= (1.83A + 0.472A/4)
2
x 0.080Ω = 304 mW (19)
Duty cycle is not included in this power calculation since the on-time duty cycle is typically <5% when in current
limit.
C
OUT
: The output capacitor should typically be no smaller than 3.3 µF, although that is dependent on the
frequency and the desired output characteristics. C
OUT
should be a low ESR good quality ceramic capacitor.
Experimentation is usually necessary to determine the minimum value for C
OUT
, as the nature of the load may
require a larger value. A load which creates significant transients requires a larger value for C
OUT
than a non-
varying load.
C
IN
and C
BYP
: The purpose of C
IN
is to supply most of the switch current during the on-time, and limit the voltage
ripple at V
IN
, since it is assumed the voltage source feeding V
IN
has some amount of source impedance. When
the buck switch turns on, the current into V
IN
suddenly increases to the lower peak of the inductor’s ripple
current, then ramps up to the upper peak, then drops to zero at turn-off. The average current during the on-time
is the average load current. For a worst case calculation, C
IN
must supply this average load current during the
maximum on-time, without letting the voltage at the VIN pin drop below a minimum operating level of 5.5V. For
this exercise 0.5V is chosen as the maximum allowed input ripple voltage. Using the maximum load current, the
minimum value for C
IN
is calculated from:
(20)
where t
ON
is the maximum on-time, and ΔV is the allowable ripple voltage at V
IN
. The purpose of C
BYP
is to
minimize transients and ringing due to long lead inductance leading to the VIN pin. A low ESR 0.1 µF ceramic
chip capacitor is recommended, and C
BYP
must be located close to the VIN and SGND pins.
C
BST
: The recommended value for C
BST
is 0.1 µF. A high quality ceramic capacitor with low ESR is
recommended as C
BST
supplies a surge current to charge the buck switch gate at each turn-on. A low ESR also
helps ensure a complete recharge during each off-time.
16 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1