LM48520TLBD National Semiconductor, LM48520TLBD Datasheet - Page 12

• Manufacturer Part Number: LM48520TLBD
• Description: BOARD EVALUATION FOR LM48520TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM48520TLBD

• Amplifier Type: Class D
• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 1.3W x 2 @ 8 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM48520
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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coefficient dielectrics, such as tantalum or aluminum elec-
trolytic. Capacitors with high-voltage coefficients, such as
ceramics, may result in increased distortion at low frequen-
cies. Other factors to consider when designing the input filter
include the constraints of the overall system. Although high
fidelity audio requires a flat frequency response between
20Hz and 20kHz, portable devices such as cell phones may
only concentrate on the frequency range of the frequency
range of the spoken human voice (typically 300Hz to 4kHz).
In addition, the physical size of the speakers used in such
portable devices limits the low frequency response; in this
case, frequencies below 150Hz may be filtered out.
SELECTING OUTPUT CAPACITOR (C
CONVERTER
A single 100µF low ESR tantalum capacitor provides suffi-
cient output capacitance for most applications. Higher capac-
itor values improve line regulation and transient response.
Typical electrolytic capacitors are not suitable for switching
converters that operate above 500kHz because of significant
ringing and temperature rise due to self-heating from ripple
current. An output capacitor with excessive ESR reduces
phase margin and causes instability.
SELECTING INPUT CAPACITOR (Cs1) FOR BOOST
CONVERTER
An input 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 is the best choice. We recommend a nomi-
nal value of 2.2µF, 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 SOFTSTART (C
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
off between the wake-up time and the startup transient cur-
rent. Using a larger capacitor value will increase wake-up time
and decrease startup transient current while the apposite ef-
fect happens with a smaller capacitor value. A general guide-
line is to use a capacitor value 1000 times smaller than the
output capacitance of the boost converter (C
start capacitor is recommended for a typical application.
SETTING THE OUTPUT VOLTAGE (V
CONVERTER
The output voltage is set using the external resistors R1 and
R2 (see Figure 1). A value of approximately 13.3kΩ is rec-
ommended for R2 to establish a divider current of approxi-
mately 92µA. R1 is calculated using the formula:
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM48520's internal Boost converter is internally
compensated, the external feed-forward capacitor C
quired for stability (see Figure 1). Adding this capacitor puts
a zero in the loop response of the converter. The recom-
mended frequency for the zero fz should be approximately
6kHz. C
f
1 can be calculated using the formula:
R1 = R2 X (V
1
/1.23 − 1)
SS
) CAPACITOR
SS
) value presents a trade
1
O
) OF BOOST
) FOR BOOST
O
). A 0.1uF soft-
f
is re-
(4)
12
SELECTING DIODES FOR BOOST
The external diode used in Figure 1 should be a Schottky
diode. A 20V diode such as the MBRS320T3 is recommend-
ed.
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:
This applies for continuous mode operation.
SELECTING INDUCTOR VALUE
Inductor value involves trade-offs in performance. Larger in-
ductors 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:
Where “lp” is the peak inductor current. The LM48520 will limit
its switch current based on peak current. With I
creasing L will increase the maximum amount of power avail-
able 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 con-
verter 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 al-
lowing 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 con-
tinuous 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 main-
tained at the typical load current values.
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the current
limiter cuts in is dependent on duty cycle of the application.
This is illustrated in a graph in the typical performance char-
acterization section which shows typical values of switch
current as a function of effective (actual) duty cycle.
CALCULATING OUTPUT CURRENT OF BOOST
CONVERTER (I
As shown in Figure 2 which depicts inductor current, the load
current is related to the average inductor current by the rela-
tion:
Duty Cycle = V
AMP
C
OUT
f
= 1 / (2 X R1 X fz)
)
+ V
E = L/2 X (I
DIODE
- V
IN
P
)
/ V
2
OUT
+ V
DIODE
P
fixed, in-
- V
SW
(5)