LM25011MY/NOPB National Semiconductor, LM25011MY/NOPB Datasheet - Page 11

• 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

plitude. The approximate operating frequency is calculated as
follows:
The buck switch duty cycle is approximately equal to:
When the load current is less than one half the inductor’s rip-
ple current amplitude the circuit operates in discontinuous
conduction mode. The off-time is longer than in continuous
conduction mode while the inductor current is zero, causing
the switching frequency to reduce as the load current is re-
duced. Conversion efficiency is maintained at light loads
since the switching losses are reduced with the reduction in
load and frequency. The approximate discontinuous operat-
ing frequency can be calculated as follows:
where R
The output voltage is set by the two feedback resistors
(R
age is calculated as follows:
Ripple voltage, which is required at the input of the regulation
comparator for proper output regulation, is generated inter-
nally in the LM25011, and externally when the LM25011A is
used. In the LM25011 the ERM (Emulated Ripple Mode) con-
trol circuit generates the required internal ripple voltage from
the ripple waveform at the CS pin. The LM25011A, which is
designed for higher frequency operation, requires additional
ripple voltage, which must be generated externally and pro-
vided to the FB pin. This is described in the Applications
Information section.
On-Time Timer
The on-time for the LM25011/LM25011A is determined by the
R
The inverse relationship with V
frequency as V
duction mode switching frequency (F
determined from the following:
T
FB1
resistor and the input voltage (V
, R
L
FB2
V
= the load resistance, and L1 is the circuit’s inductor.
OUT
in the Block Diagram). The regulated output volt-
= 2.51V x (R
IN
is varied. To set a specific continuous con-
FB1
IN
+ R
results in a nearly constant
FB2
IN
), calculated from:
) / R
S
), the R
FB1
T
resistor is
(1)
(2)
(3)
(4)
(5)
(6)
11
The on-time must be chosen greater than 90 ns for proper
operation. Equations 1, 5 and 6 are valid only during normal
operation - i.e., the circuit is not in current limit. When the
LM25011 operates in current limit, the on-time is reduced by
≊
reduces the peak inductor current which may be excessively
high if the load current and the input voltage are simultane-
ously high. This feature operates on a cycle-by-cycle basis
until the load current is reduced and the output voltage re-
sumes its normal regulated value. The maximum continuous
current into the RT pin must be less than 2 mA. For high fre-
quency applications, the maximum switching frequency is
limited at the maximum input voltage by the minimum on-time
one-shot period (90 ns). At minimum input voltage the maxi-
mum switching frequency is limited by the minimum off-time
one-shot period, which, if reached, prevents achievement of
the proper duty cycle.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the voltage across the external current sense resistor
R
recirculating current flows through the inductor, through the
load, through the sense resistor, and through D1 to the in-
ductor. If the voltage across the sense resistor exceeds the
threshold (V
to delay the start of the next on-time period. The next on-time
starts when the recirculating current decreases such that the
voltage across R
FB is below 2.51V. The operating frequency is typically lower
due to longer-than-normal off-times. When current limit is de-
tected, the on-time is reduced by
the voltage at the FB pin is below its threshold when the volt-
age across R
current limiting).
Figure 2 illustrates the inductor current waveform during nor-
mal operation and in current limit. During the first “Normal
Operation” the load current is I
current waveform. As the load resistance is reduced, the in-
ductor current increases until the lower peak of the inductor
ripple current exceeds the threshold. During the “Current Lim-
ited” portion of Figure 2, each on-time is reduced by
resulting in lower ripple amplitude for the inductor’s current.
During this time the LM25011 is in a constant current mode
with an average load current equal to the current limit thresh-
old plus half the ripple amplitude (I
is below the normal regulated value. Normal operation re-
sumes when the load current is reduced (to I
V
that in the second period of “Normal Operation”, even though
the inductor’s peak current exceeds the current limit threshold
during part of each cycle, the circuit is not in current limit since
the inductor current falls below the current limit threshold dur-
ing each off time. The peak current allowed through the buck
switch is 3.5A, and the maximum allowed average current is
2.0A.
OUT
40% (this feature is not present in LM25011A). This feature
S
. Referring to the Block Diagram, during the off-time the
and the on-time to return to their normal values. Note
ILIM
S
) the current limit comparator output switches
reduces to its threshold (V
S
reduces to the threshold and the voltage at
01
, the average of the inductor
OCL
≊
40% (only in LM25011) if
), and the output voltage
OUT
is low due to
O2
www.national.com
), allowing
≊
40%,