FB

SW

L1

C2

R3

SM74301

F =

1.385 x 10

SM74301

www.ti.com

SNVS719A – JULY 2011

where

•

(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)

regulation point can be calculated as follows:

(3)

The SM74301 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 SM74301. 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

The high voltage bias regulator is integrated within the SM74301. 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

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

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 turn-on sequence is shown in Figure 10. During the initial delay (t1) VCC ramps up at a rate determined by

voltage lock-out (UVLO, typically 5.3V) the buckswitch is enabled. The inductor current increases to the current

Copyright © 2011, Texas Instruments Incorporated

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Product Folder Links: SM74301