LM48520, LM48520TLBD

SNAS367C – FEBRUARY 2008 – REVISED APRIL 2013

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Selecting Soft-Start (CSS) Capacitor

The soft-start function charges the boost converter reference voltage slowly. This allows the output of the boost

converter to ramp up slowly thus limiting the transient current at startup. Selecting a soft-start capacitor (CSS)

value presents a trade off between the wake-up time and the startup transient current. Using a larger capacitor

value will increase wake-up time and decrease startup transient current while the apposite effect happens with a

smaller capacitor value. A general guideline is to use a capacitor value 1000 times smaller than the output

capacitance of the boost converter (CO). A 0.1uF soft-start capacitor is recommended for a typical application.

Setting the Output Voltage (V1) of boost Converter

The output voltage is set using the external resistors R1 and R2 (see Figure 1). A value of approximately 13.3k

Ω

is recommended for R2 to establish a divider current of approximately 92µA. R1 is calculated using the formula:

R1 = R2 X (V1/1.23 − 1)

(4)

Feed-Forward Compensation for Boost Converter

Although the LM48520's internal Boost converter is internally compensated, the external feed-forward capacitor

Cf is required for stability (see Figure 1). Adding this capacitor puts a zero in the loop response of the converter.

The recommended frequency for the zero fz should be approximately 6kHz. Cf1 can be calculated using the

formula:

Cf = 1 / (2 X R1 X fz)

(5)

Selecting Diodes for Boost

The external diode used in Figure 1 should be a Schottky diode. A 20V diode such as the MBRS320T3 is

recommended.

The MBRS320T3 series of diodes are designed to handle a maximum average current of 3A.

Duty Cycle

The maximum duty cycle of the boost converter determines the maximum boost ratio of output-to-input voltage

that the converter can attain in continuous mode of operation. The duty cycle for a given boost application is

defined as:

Duty Cycle = VOUT + VDIODE - VIN/ VOUT + VDIODE - VSW

(6)

This applies for continuous mode operation.

Selecting Inductor Value

Inductor value involves trade-offs in performance. Larger inductors reduce inductor ripple current, which typically

means less output voltage ripple (for a given size of output capacitor). Larger inductors also mean more load

power can be delivered because the energy stored during each switching cycle is:

E = L/2 X (IP)

2

(7)

Where “lp” is the peak inductor current. The LM48520 will limit its switch current based on peak current. With IP

fixed, increasing L will increase the maximum amount of power available to the load. Conversely, using too little

inductance may limit the amount of load current which can be drawn from the output. Best performance is usually

obtained when the converter is operated in “continuous” mode at the load current range of interest, typically

giving better load regulation and less output ripple. Continuous operation is defined as not allowing the inductor

current to drop to zero during the cycle. Boost converters shift over to discontinuous operation if the load is

reduced far enough, but a larger inductor stays continuous over a wider load current range.

During the TBDµs ON-time, the inductor current ramps up TBDA and ramps down an equal amount during the

OFF-time. This is defined as the inductor “ripple current”. It can also be seen that if the load current drops to

about TBDmA, the inductor current will begin touching the zero axis which means it will be in discontinuous

mode. A similar analysis can be performed on any boost converter, to make sure the ripple current is reasonable

and continuous operation will be maintained at the typical load current values.

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