lm25011myx National Semiconductor Corporation, lm25011myx Datasheet - Page 12

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

www.national.com
Power Good Output (PGD)
The Power Good output (PGD) indicates when the voltage at
the FB pin is close to the internal 2.51V reference voltage.
The rising threshold at the FB pin for the PGD output to switch
high is 95% of the internal reference. The falling threshold for
the PGD output to switch low is approximately 3.3% below the
rising threshold.
The PGD pin is internally connected to the drain of an N-
channel MOSFET switch. An external pull-up resistor
(R
7V, is required at PGD to indicate the LM25011’s status to
other circuitry. When PGD is low, the pin’s voltage is deter-
mined by the current into the pin. See the graph “PGD Low
Voltage vs. Sink Current”.
Upon powering up the LM25011, the PGD pin is high until the
voltage at V
As V
takes the voltage at the FB pin above 95% of the internal ref-
erence voltage, at which time PGD switches high. As V
decreased (during shutdown) PGD remains high until either
the voltage at the FB pin falls below
erence, or when V
whichever occurs first. PGD then switches low, and remains
low until V
If the LM25011 is used as a tracking regulator (see the Soft-
start section), the PGD output is high as long as the voltage
at the FB pin is above the thresholds mentioned above.
Thermal Shutdown
The LM25011 should be operated so the junction temperature
does not exceed 125°C. If the junction temperature increases
above that, an internal Thermal Shutdown circuit activates
(typically) at 155°C, taking the controller to a low power reset
state by disabling the buck switch and taking the SS pin to
ground. This feature helps prevent catastrophic failures from
accidental device overheating. When the junction tempera-
ture reduces below 135°C (typical hysteresis = 20°C) normal
operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with a design example. Referring to the Block Dia-
gram, the circuit is to be configured for the following
specifications:
•
•
•
•
•
•
R
their ratio is calculated from:
FB2
PGD
V
V
Minimum load current for continuous conduction mode
(I
Maximum load current (I
Switching frequency (F
Soft-start time = 5 ms
OUT(min)
OUT
IN
IN
and R
), connected to an appropriate voltage not exceeding
= 8V to 36V
is increased PGD stays low until the output voltage
= 5V
IN
FIGURE 4. Shutdown Implemetation
FB1
IN
falls below 2V, at which time PGD switches high.
= 300 mA
reaches 2V, at which time PGD switches low.
: These resistors set the output voltage, and
IN
falls below its lower UVLO threshold,
S
) = 1.0 MHz
OUT(max)
≊
= 1.5 A
92% of the internal ref-
30094626
IN
is
12
For this example, R
be chosen from standard value resistors in the range of 1.0
kΩ – 10 kΩ which satisfy the above ratio. For this example,
4.99 kΩ is chosen for both resistors, providing a 5.02V output.
R
ing frequency. First check that the desired frequency does not
require an on-time or off-time shorter than the minimum al-
lowed values (90 ns and 150, respectively). The minimum on-
time occurs at the maximum input voltage. For this example:
The minimum off-time occurs at the minimum input voltage.
For this example:
Both the on-time and off-time are acceptable since they are
significantly greater than the minimum value for each. The
R
input voltage:
A standard value 118 kΩ resistor is selected. The minimum
on-time calculates to 152 ns at Vin = 36V, and the maximum
on-time calculates to 672 ns at Vin = 8V
L1: The parameters controlled by the inductor are the inductor
current ripple amplitude (I
tude across the current sense resistor R
current is used to determine the maximum allowable ripple in
order to maintain continuous conduction mode (the lower
peak does not reach 0 mA). This is not a requirement of the
LM25011, but serves as a guideline for selecting L1. For this
example, the maximum ripple current should be less than:
For applications where the minimum load current is zero, a
good starting point for allowable ripple is 20% of the maximum
load current. In this case substitute 20% of I
(min)
8 is then used in the following equation:
A standard value 10 µH inductor is chosen. Using this inductor
value, the maximum ripple current amplitude, which occurs at
maximum V
is 1736 mA at maximum load current. Ensure the selected
inductor is rated for this peak current. The minimum ripple
current, which occurs at minimum V
p.
R
imum load current, using the minimum ripple current calcu-
lated above. The current limit threshold is the lower peak of
the inductor current waveform when in current limit (see Fig-
ure 2).
T
T
S
: This resistor sets the on-time, and (by default) the switch-
: The minimum current limit threshold is calculated at max-
resistor is calculated from equation 6 using the minimum
in equation 8. The ripple amplitude calculated in Equation
I
OR(max)
IN
R
, calculates to 472 mAp-p, and the peak current
FB2
I
LIM
= 2 x I
/R
= 1.5A – (0.2 A/2) = 1.4A
FB1
FB2
/R
= (V
OUT(min)
FB1
OR
OUT
), and the ripple voltage ampli-
= 0.992. R
/2.51V) - 1
= 600 mA p-p
IN
, calculates to 200 mAp-
S
FB1
. The minimum load
and R
OUT(max)
FB2
for I
should
OUT
(7)
(8)