4

4.2

4.4

4.6

4.8

5

5.2

4

4.5

5

5.5

6

6.5

7

Input Voltage (V)

1A

2A

3A

C003

17

LM53602, LM53603

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SNVSAR0 – NOVEMBER 2016

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8.4.3 Dropout

One of the parameters that influences the dropout performance of a buck regulator is the minimum off-time. As

the input voltage is reduced, to near the output voltage, the off-time of the high-side switch starts to approach the

minimum value (see Timing Requirements). Beyond this point the switching may become erratic or the output

voltage falls out of regulation. To avoid this problem, the LM53603 automatically reduces the switching frequency

to increase the effective duty cycle. This results in two specifications regarding dropout voltage, as shown in the

System Characteristics table. One specification indicates when the switching frequency drops to 1.85 MHz;

avoiding the A.M. radio band. The other specification indicates when the output voltage has fallen to 1% of

nominal. See the Application Curves for typical values of dropout. The overall dropout characteristic for the 5-V

option, can be seen in Figure 14. The SYNC input is ignored during frequency fold-back in dropout.

Figure 14. Overall Dropout Characteristic

8.4.4 Input Voltage Frequency Fold-Back

At higher input voltages the on-time of the high-side switch becomes small. When the minimum is reached (see

Timing Requirements), the switching may become erratic or the output voltage falls out of regulation. To avoid

this behavior, the LM53603 automatically reduces the switching frequency at input voltages above about 20 V

(see Application Curves). In this way the device avoids the minimum on-time restriction and maintains regulation

at abnormally high battery voltages. The SYNC input is ignored during frequency fold-back at high input voltages.