Datasheet

L1 =
V
OUT
x (V
IN
- V
OUT
)
I
OR
x F
s
x V
IN
SM72485
SNVS697D –JANUARY 2011–REVISED APRIL 2013 www.ti.com
Thermal Protection
The SM72485 should be operated so the junction temperature does not exceed 125°C during normal operation.
An internal Thermal Shutdown circuit is provided to shutdown the SM72485 in the event of a higher than normal
junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state by
disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When
the junction temperature reduces below 140°C (typical hysteresis = 25°C) normal operation is resumed.
APPLICATIONS INFORMATION
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illustrated with a design example. Refer to the Block
Diagram. The following steps will configure the SM72485 for:
• Input voltage range (Vin): 12V to 90V
• Output voltage (V
OUT1
): 10V
• Load current (for continuous conduction mode): 100 mA to 150 mA
R
FB1
, R
FB2
: V
OUT
= V
FB
x (R
FB1
+ R
FB2
) / R
FB1
, and since V
FB
= 2.5V, the ratio of R
FB2
to R
FB1
calculates as 3:1.
Standard values of 3.01 kΩ and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is
maintained.
F
s
and R
T
: The recommended operating frequency range for the SM72485 is 50 kHz to 1.1 MHz. Unless the
application requires a specific frequency, the choice of frequency is generally a compromise since it affects the
size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of
400 ns, is calculated from:
F
MAX
= V
OUT
/ (V
INMAX
x 400 ns) (6)
For this exercise, Fmax = 277 kHz. From Equation 2, R
T
calculates to 260 kΩ. A standard value 309 kΩ resistor
will be used to allow for tolerances in Equation 2, resulting in a frequency of 234 kHz.
L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor
value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum
ripple current occurs at maximum Vin.
a) Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude
(I
OR
) must be less than 200 mA p-p so the lower peak of the waveform does not reach zero. L1 is calculated
using the following equation:
(7)
At Vin = 90V, L1(min) calculates to 190 µH. The next larger standard value (220 µH) is chosen and with this
value I
OR
calculates to 173 mA p-p at Vin = 90V, and 32 mA p-p at Vin = 12V.
b) Maximum load current: At a load current of 150 mA, the peak of the ripple waveform must not reach the
minimum ensured value of the SM72485’s current limit threshold (240 mA). Therefore the ripple amplitude must
be less than 180 mA p-p, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum Vin
and Io, the peak of the ripple will be 236 mA. While L1 must carry this peak current without saturating or
exceeding its temperature rating, it also must be capable of carrying the maximum specified value of the
SM72485’s current limit threshold (360 mA) without saturating, since the current limit is reached during startup.
The DC resistance of the inductor should be as low as possible to minimize its power loss.
C3: The capacitor on the V
CC
output provides not only noise filtering and stability, but its primary purpose is to
prevent false triggering of the V
CC
UVLO at the buck switch on/off transitions. C3 should be no smaller than 0.47
µF.
C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its
ESR, ripple voltage due to its capacitance, and the nature of the load.
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