Datasheet

L1 =
5V x (75V ± 5V)
0.5A x 300 kHz x 75V
= 31 PH
( )
(
)
( )
OUT OUT
IN max
RIPPLE S
IN max
V V V
L1
I F V
´ -
=
´ ´
I
PK+
L1 Current
0 mA
I
PK-
I
O
I
RIPPLE
1/Fs
9
3
T
12
1
580 10
300 10
R
135 10
-
-
æ ö
æ ö
- ´
ç ÷
ç ÷
´
è ø
è ø
=
´
LM5576, LM5576-Q1
SNVS447I –JANUARY 2007–REVISED APRIL 2013 www.ti.com
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction
temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power reset state,
disabling the output driver and the bias regulator. This feature is provided to prevent catastrophic failures from
accidental device overheating.
Application Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. The Bill of
Materials for this design is listed in Table 1. The circuit shown in Figure 9 is configured for the following
specifications:
• V
OUT
= 5V
• V
IN
= 7V to 75V
• Fs = 300kHz
• Minimum load current (for CCM) = 250mA
• Maximum load current = 3A
R3 (R
T
)
R
T
sets the oscillator switching frequency. Generally, higher frequency applications are smaller but have higher
losses. Operation at 300kHz was selected for this example as a reasonable compromise for both small size and
high efficiency. The value of R
T
for 300kHz switching frequency can be calculated as follows:
(7)
The nearest standard value of 21kΩ was chosen for R
T
.
L1
The inductor value is determined based on the operating frequency, load current, ripple current, and the
minimum and maximum input voltage (V
IN(min)
, V
IN(max)
).
Figure 16. Inductor Current Waveform
To keep the circuit in continuous conduction mode (CCM), the maximum ripple current I
RIPPLE
should be less
than twice the minimum load current, or 0.5Ap-p. Using this value of ripple current, the value of inductor (L1) is
calculated using the following:
(8)
(9)
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