LM43601PWPR datasheet(21/57 Pages) TI2

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RFBT

RFBB

CFF

VOUT

10000

100000

1000000

5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

VIN (V)

Load = 0.01 A

Load = 0.1 A

Load = 0.5 A

Load = 1 A

C001

LM43601

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SNVSA44B – AUGUST 2014 – REVISED JANUARY 2018

Product Folder Links: LM43601

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Feature Description (continued)

Figure 42. Switching Frequency Decreases in Dropout Operation

VOUT = 5 V, FS = 1 MHz

7.3.9 Internal Compensation and CFF

The LM43601 is internally compensated with RC = 400 kΩ and CC = 50 pF as shown in Functional Block

Diagram. The internal compensation is designed such that the loop response is stable over the entire operating

frequency and output voltage range. Depending on the output voltage, the compensation loop phase margin can

be low with all ceramic capacitors. TI recommends an external feed-forward capacitor, CFF, be placed in parallel

with the top resistor divider, RFBT, for optimum transient performance.

Figure 43. Feed-Forward Capacitor for Loop Compensation

The feed-forward capacitor CFF in parallel with RFBT places an additional zero before the cross over frequency of

the control loop to boost phase margin. The zero frequency can be found by

fZ-CFF = 1 / (2π × RFBT × CFF)

(8)

An additional pole is also introduced with CFF at the frequency of

fP-CFF = 1 / (2π × CFF × ( RFBT // RFBB))

(9)

Select the CFF so that the bandwidth of the control loop without the CFF is centered between fZ-CFF and fP-CFF. The

zero fZ-CFF adds phase boost at the crossover frequency and improves transient response. The pole fP-CFF helps

maintaining proper gain margin at frequency beyond the crossover.

Designs with different combinations of output capacitors need different CFF. Different types of capacitors have

different equivalent series resistance (ESR). Ceramic capacitors have the smallest ESR and require the most

CFF. Electrolytic capacitors have much larger ESR and the ESR zero frequency

fZ-ESR = 1 / (2π × ESR × COUT)

(10)

would be low enough to boost the phase up around the crossover frequency. Designs using mostly electrolytic

capacitors at the output may not require any CFF.

LM43601PWPR Datasheet(PDF) 21 Page - Texas Instruments

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