Datasheet

FB
SW
L1
C2
R
FB2
V
OUT2
R3
SM72485
R
FB1
F =
V
OUT
1.385 x 10
-10
x R
T
F =
V
OUT
2
x L x 1.04 x 10
20
R
L
x (R
T
)
2
SM72485
www.ti.com SNVS697D –JANUARY 2011–REVISED APRIL 2013
where
• R
L
= the load resistance (1)
In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.
In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively
constant with load and line variations. The approximate continuous mode operating frequency can be calculated
as follows:
(2)
The output voltage (V
OUT
) is programmed by two external resistors as shown in the Block Diagram. The
regulation point can be calculated as follows:
V
OUT
= 2.5 x (R
FB1
+ R
FB2
) / R
FB1
(3)
The SM72485 regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor C2. A minimum of 25mV to 50mV of ripple voltage at the feedback pin
(FB) is required for the SM72485. In cases where the capacitor ESR is too small, additional series resistance
may be required (R3 in the Block Diagram).
For applications where lower output voltage ripple is required the output can be taken directly from a low ESR
output capacitor, as shown in Figure 9. However, R3 slightly degrades the load regulation.
Figure 9. Low Ripple Output Configuration
Start-Up Regulator (V
CC
)
The high voltage bias regulator is integrated within the SM72485. The input pin (VIN) can be connected directly
to line voltages between 6V and 95V, with transient capability to 100V. Referring to the block diagram and the
graph of V
CC
vs V
IN
, when V
IN
is between 6V and the bypass threshold (nominally 8.5V), the bypass switch (Q2)
is on, and V
CC
tracks V
IN
within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100Ω, with
inherent current limiting at approximately 100 mA. When V
IN
is above the bypass threshold Q2 is turned off, and
V
CC
is regulated at 7V. The V
CC
regulator output current is limited at approximately 9.2 mA. When the SM72485
is shutdown using the RT/SD pin, the V
CC
bypass switch is shut off regardless of the voltage at V
IN
.
When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 - 3 µs. The
capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its
absolute maximum rating in response to a step input applied at V
IN
. C3 must be located as close as possible to
the VCC and RTN pins. In applications with a relatively high input voltage, power dissipation in the bias regulator
is a concern. An auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin to shut off
the V
CC
regulator, thereby reducing internal power dissipation. The current required into the VCC pin is shown in
the graph “I
CC
Current vs. Applied V
CC
Voltage”. Internally a diode connects VCC to VIN requiring that the
auxiliary voltage be less than V
IN
.
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