LM34917A Datasheet, PDF(13/16 Page) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

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LM34917A Datasheet, PDF (13/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

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20216634

FIGURE 6. Frequency vs. VIN (Circuit of Figure 4)

ALTERNATE OUTPUT RIPPLE CONFIGURATIONS

For applications which can accept higher levels of ripple at

VOUT, the following configurations are simpler and a bit more

economical.

a) Alternate #1: In Figure 7 R3, C7 and C8 are removed, and

Cff and R4 are installed, resulting in a higher ripple level than

the circuit of Figure 4. Ripple is created at VOUT by the

inductorâs ripple current passing through R4. That ripple volt-

age is AC coupled to the FB pin through Cff, allowing the

minimum ripple at VOUT to be set at 25 mVp-p. The minimum

ripple current amplitude (IOR(min)) is calculated by re-arranging

Equation 7 using tON(max) and VIN(min). The minimum value for

R4 is calculated from:

circuits of Figure 4 and Figure 7. Ripple is created at VOUT by

the inductorâs ripple current passing through R4. That ripple

voltage is coupled to the FB pin through the feedback resis-

tors (R1, R2). Since the LM34917A requires a minimum of 25

mVp-p ripple at the FB pin, the ripple required at VOUT is high-

er than 25 mVp-p by the gain of the feedback resistors. The

minimum ripple current (IOR(min)) is calculated by re-arranging

Equation 7 using tON(max) and VIN(min). The minimum value for

R4 is calculated from:

The next larger standard value resistor should be used for R4.

The next larger standard value resistor should be selected for

R4 to allow for tolerances. The minimum value for Cff is de-

termined from:

The next larger standard value capacitor should be used for

Cff.

20216627

FIGURE 8. Maximum Ripple Configuration

c) Alternate minimum ripple configuration: The circuit in

Figure 9 is the same as that in Figure 8, except the output

voltage is taken from the junction of R4 and C2. The ripple at

VOUT is determined by the inductorâs ripple current and C2âs

characteristics. However, R4 slightly degrades the load reg-

ulation. This circuit may be suitable if the load current is fairly

constant. R4 is calculated as described in Alternate #2 above.

20216626

FIGURE 7. Reduced Ripple Configuration

b) Alternate #2: In Figure 8, R3, C7 and C8 are removed,

and R4 is installed, resulting in a higher ripple level than the

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