Datasheet

LM48511
www.ti.com SNAS416G –JULY 2007–REVISED MAY 2013
SELECTING REGULATING BYPASS CAPACITOR
A supply bypass capacitor is required to serve as an energy reservoir for the current which must flow into the coil
each time the switch turns on. This capacitor must have extremely low ESR, so ceramic capacitors are the best
choice. A nominal value of 10μF is recommended, but larger values can be used. Since this capacitor reduces
the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line
to other circuitry.
SELECTING THE SOFTSTART (C
SS
) 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 (C
SS
)
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 (C2). A 0.1uF soft-start capacitor is recommended for a typical application.
The following table shows the relationship between C
SS
start-up time and surge current.
C
SS
Boost Set-up Time Input Surge Current
(μF) (ms) (mA)
0.1 5.1 330
0.22 10.5 255
0.47 21.7 220
V
DD
= 5V, PV
1
= 7.8V (continuous mode)
SELECTING DIODE (D1)
Use a Schottkey diode, as shown in Figure 1. A 30V diode such as the DFLS230LH from Diodes Incorporated is
recommended. The DFLS230LH diodes are designed to handle a maximum average current of 2A.
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 by:
Duty Cycle = (PV1+V
D
-V
DD
) / (PV1+V
D
-V
SW
) (5)
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 (I
P
)
2
Where
• “I
P
” is the peak inductor current (6)
The LM48511 will limit its switch current based on peak current. With I
P
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.
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