lm25011myx National Semiconductor Corporation, lm25011myx Datasheet - Page 13

• Manufacturer Part Number: lm25011myx
• Description: 42v, 2a Constant On-time Switching Regulator With Adjustable Current Limit
• Manufacturer: National Semiconductor Corporation

Current limit detection occurs when the voltage across the
sense resistor (R
low for tolerances, the sense resistor value is calculated using
the minimum threshold specification:
The next smaller standard value, 80 mΩ, is selected. The next
step is to ensure that sufficient ripple voltage occurs across
R
Requirements section, a minimum of 10mVp-p voltage ripple
is required across the R
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:
If the ripple voltage had calculated to less than 10 mVp-p the
inductor value would have to be reduced to increase the ripple
current amplitude. This would have required a recalculation
of I
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 cur-
rent. Under normal load conditions, the maximum power dis-
sipation in R
maximum input voltage where the on-time duty cycle is min-
imum. In this design example, the minimum on-time duty
cycle is:
At maximum load current, the power dissipation in R
to:
When in current limit the maximum power dissipation in R
calculates to
Duty cycle is not included in this power calculation since the
on-time duty cycle is typically <5% when in current limit.
C
3.3 µF, although that is dependent on the frequency and the
desired output characteristics. C
good quality ceramic capacitor. Experimentation is usually
necessary to determine the minimum value for C
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for
C
C
switch current during the on-time, and limit the voltage ripple
S
OUT
OUT
IN
with this value sense resistor. As mentioned in the Ripple
LIM
and C
: The output capacitor should typically be no smaller than
than a non-varying load.
P
P
V
and R
(RS)
(RS)
RIPPLE
BYP
= (1.5A)
= (1.83A + 0.472A/4)
S
S
: The purpose of C
= ΔI x R
in the above equations. Since the minimum
R
occurs at maximum load current, and at
S
S
) reaches the current limit threshold. To al-
2
= 115 mV/1.4A = 82 mΩ
x 0.080Ω x (1 – 0.139) = 155 mW
S
S
= 200 mA x 0.080Ω = 16 mV
sense resistor during the off-time to
2
OUT
IN
x 0.080Ω = 304 mW
is to supply most of the
should be a low ESR
OUT
S
is equal
, as the
S
13
at V
some amount of source impedance. When the buck switch
turns on, the current into V
peak of the inductor’s ripple current, then ramps up to the up-
per 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
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
where t
ripple voltage at V
sients and ringing due to long lead inductance leading to the
VIN pin. A low ESR 0.1 µF ceramic chip capacitor is recom-
mended, and C
SGND pins.
C
quality ceramic capacitor with low ESR is recommended as
C
at each turn-on. A low ESR also helps ensure a complete
recharge during each off-time.
C
time, i.e. the time for the output voltage to reach its final value
(t1 in Figure 1). For a soft-start time of 5 ms, the capacitor
value is determined from the following:
D1: A Schottky diode is recommended. Ultra-fast recovery
diodes are not recommended as the high speed transitions at
the SW pin may affect the regulator’s operation due to the
diode’s reverse recovery transients. The diode must be rated
for the maximum input voltage, the maximum load current,
and the peak current which occurs when the current limit and
maximum ripple current are reached simultaneously. The
diode’s average power dissipation is calculated from:
where V
time duty cycle.
FINAL CIRCUIT
The final circuit is shown in Figure 5, and its performance is
shown in Figure 6 and Figure 7. The current limit measured
approximately 1.62A at Vin = 8V, and 1.69A at Vin = 36V.
BST
BST
SS
: The capacitor at the SS pin determines the soft-start
IN
: The recommended value for C
supplies a surge current to charge the buck switch gate
, since it is assumed the voltage source feeding V
ON
F
is the diode’s forward voltage drop, and D is the on-
is the maximum on-time, and ΔV is the allowable
IN
is calculated from:
BYP
IN
P
IN
. The purpose of C
D1
must be located close to the VIN and
must supply this average load current
= V
F
IN
x I
suddenly increases to the lower
OUT
x (1 - D)
BYP
BST
is to minimize tran-
is 0.1 µF. A high
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IN
has
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