Datasheet

R
T
=
V
OUT
- (V
IN
x F
S
x 15 ns)
F
S
x 4.1 x 10
-11
- 500:
t
ON
=
(V
IN
)
4.1 x 10
-11
x (R
T
+ 500:)
+ 15 ns
LM25011, LM25011-Q1
www.ti.com SNVS617G –APRIL 2009–REVISED FEBRUARY 2013
Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is
generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM
(Emulated Ripple Mode) control 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 provided 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
T
resistor and the input voltage (V
IN
), calculated
from:
(5)
The inverse relationship with V
IN
results in a nearly constant frequency as V
IN
is varied. To set a specific
continuous conduction mode switching frequency (F
S
), the R
T
resistor is determined from the following:
(6)
The on-time must be chosen greater than 90 ns for proper operation. Equation 1, Equation 5, and Equation 6 are
valid only during normal operation; that is, the circuit is not in current limit. When the LM25011 operates in
current limit, the on-time is reduced by approximately 40% (this feature is not present in LM25011A). This feature
reduces the peak inductor current which may be excessively high if the load current and the input voltage are
simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the
output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be
less than 2 mA. For high frequency 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 maximum 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 monitoring the voltage across the external current sense
resistor R
S
. Referring to the Block Diagram, during the off-time the recirculating current flows through the
inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the
sense resistor exceeds the threshold (V
ILIM
) the current limit comparator output switches 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
S
reduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically
lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by
approximately 40% (only in LM25011) if the voltage at the FB pin is below its threshold when the voltage across
R
S
reduces to its threshold (V
OUT
is low due to current limiting).
Figure 18 illustrates the inductor current waveform during normal operation and in current limit. During the first
“Normal Operation” the load current is I
01
, the average of the inductor current waveform. As the load resistance is
reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold.
During the “Current Limited” portion of Figure 18, each on-time is reduced by approximately 40%, 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 threshold plus half the ripple amplitude (I
OCL
), and the output
voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to I
O2
),
allowing V
OUT
and the on-time to return to their normal values. Note 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 during each off
time. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is
2.0A.
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