LM25011MY/NOPB National Semiconductor, LM25011MY/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM25011MY/NOPB
• Description: IC BUCK ADJ 2A 10MSOP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM25011MY/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.51 ~ 40 V
• Current - Output: 2A
• Frequency - Switching: Up to 2MHz
• Voltage - Input: 6 ~ 42 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Power - Output: 155mW
• Primary Input Voltage: 42V
• No. Of Outputs: 1
• Output Voltage: 40V
• Output Current: 2A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 3 - 168 Hours
• Filter Terminals: SMD
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM25011MYTR

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FIGURE 8. Option A – Lowest Cost Ripple Configuration
Option B) Intermediate Ripple Configuration: This config-
uration generates less ripple at V
the addition of one capacitor (Cff), as shown in Figure 9.
FIGURE 9. Option B – Intermediate Ripple Configuration
Since the output ripple is passed by Cff to the FB pin with little
or no attenuation, R1’s value can be chosen so the minimum
ripple at V
calculated from:
where ΔI is the minimum ripple current amplitude, which oc-
curs at minimum Vin. The minimum value for Cff is calculated
from:
where t
R
Option C) Minimum Ripple Configuration:
FB1
//R
ON(max)
FB2
OUT
is the parallel equivalent of the feedback resistors.
is the maximum on-time (at minimum V
is
≊
150 mVp-p. The minimum value for R1 is
OUT
than Option A above by
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IN
), and
16
In some applications, the ripple induced by series resistor R1
may not be acceptable. An external ripple circuit, as shown in
Figure 10, can be used to provide the required ripple to the
FB pin.
1.
2.
3.
PC BOARD LAYOUT
The LM25011 regulation and current limit comparators are
very fast, and respond to short duration noise pulses. Layout
considerations are therefore critical for optimum perfor-
mance. The layout must be as neat and compact as possible,
and all of the components must be as close as possible to
their associated pins. The two major current loops conduct
currents which switch very fast, and therefore those loops
must be as small as possible to minimize conducted and ra-
diated EMI. The first loop is formed by C
to SW pins, L1, C
loop is formed by R
ground connection from CSG to the ground end of C
be as short and direct as possible.
The power dissipation within the LM25011 can be approxi-
mated by determining the circuit’s total conversion loss (P
P
wheeling diode, the sense resistor, and the inductor. The
power loss in the diode is approximately:
where Iout is the load current, V
age drop, and D is the on-time duty cycle. The power loss in
the sense resistor is:
The power loss in the inductor is approximately:
where R
is an approximation for the AC losses. If it is expected that the
internal dissipation of the LM25011 will produce excessive
junction temperatures during normal operation, good use of
OUT
The time constant τ=Rr*Cr should be greater than 8-10
times the switching period to generate a triangular ramp
at FB pin.
The smallest ripple at feedback ΔVFB = (VIN(min)-
VOUT)*TON(max)/τ.
The ramp capacitor Cr should much smaller than the ac
coupling capacitor Cac. Usually Cac=100nF, Cr=1nF,
and Rr is chosen to satisfy conditions 1 and 2 above.
), and then subtracting the power losses in the free-
FIGURE 10. Option C: Minimum Output Ripple
L
is the inductor’s DC resistance, and the 1.1 factor
P
RS
OUT
P
P
D1
= (I
L1
S
, and back to C
, D1, L1, C
Configuration
= I
= I
OUT
OUT
OUT
)
2
2
x V
x R
x R
F
F
S
is the diode’s forward volt-
OUT
L
x (1-D)
x (1 – D)
x 1.1
IN
and back to R
. The second current
IN
, through the VIN
IN
should
S
. The
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IN
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