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20

LM53602, LM53603

SNVSAR0 – NOVEMBER 2016

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Copyright © 2016, Texas Instruments Incorporated

(1)

16V X7R capacitors used : C3225X7R1C226M250AC (TDK)

(3)

some cases are given in the table below. It is important to note that these values provide a first approximation

only and need to be verified for each application by the designer.

(1)

L

3.2V

44µF

2.2µH

69.8kΩ

31.6kΩ

33pF

3.2V

110µF

2.2µH

69.8kΩ

31.6kΩ

120pF

5.1V

44µF

2.2µH

80.6kΩ

19.6kΩ

33pF

5.1V

110µF

2.2µH

80.6kΩ

19.6kΩ

220pF

8V

66µF

4.7µH

86.6kΩ

12.4kΩ

120pF

8V

100µF

4.7µH

86.6kΩ

12.4kΩ

220pF

10V

66µF

4.7µH

90.9kΩ

10.0kΩ

120pF

9.2.1.2.2

Output Capacitors

The LM53603 is designed to work with low-ESR ceramic capacitors. The effective value of these capacitors is

defined as the actual capacitance under voltage bias and temperature. All ceramic capacitors have a large

voltage coefficient, in addition to normal tolerances and temperature coefficients. Under DC bias, the capacitance

value drops considerably. Larger case sizes or higher voltage capacitors are better in this regard. To help

mitigate these effects, multiple small capacitors can be used in parallel to bring the minimum effective

capacitance up to the desired value. This can also ease the RMS current requirements on a single capacitor.

Table 4 shows the nominal and minimum values of total output capacitance recommended for the LM53603. The

values shown also provide a starting point for other output voltages, when using the ADJ option. Also shown are

the measured values of effective capacitance for the indicated capacitor. More output capacitance can be used

to improve transient performance and reduce output voltage ripple.

In practice, the output capacitor has the most influence on the transient response and loop phase margin. Load

transient testing and Bode plots are the best way to validate any given design, and should always be completed

before the application goes into production. A careful study of temperature and bias voltage variation of any

candidate ceramic capacitor should be made to ensure that the minimum value of effective capacitance is

provided. The best way to obtain an optimum design is to use the Texas Instruments WEBENCH Design Tool.

optimizing the design. Application report Optimizing Transient Response of Internally Compensated dc-dc

Converters With Feedforward Capacitor (SLVA289) should prove helpful when adjusting the feed-forward

capacitor.

In addition to the capacitance shown in Table 4, a small ceramic capacitor placed on the output can help to

reduce high frequency noise. Small case size ceramic capacitors in the range of 1 nF to 100 nF can be very

helpful in reducing spikes on the output caused by inductor parasitics.

The maximum value of total output capacitance should be limited to between 300 µF and 400 µF. Large values

of output capacitance can prevent the regulator from starting-up correctly and adversely effect the loop stability. If

values in the range given above, or greater, are to be used, then a careful study of start-up at full load and loop

stability must be performed.