LM48510SDBD  

Manufacturer Part Number  LM48510SDBD 
Description  BOARD EVALUATION LM48510SD 
Manufacturer  National Semiconductor 
Series  Boomer® 
LM48510SDBD datasheet 

Specifications of LM48510SDBD  

Amplifier Type  Class D  Output Type  1Channel (Mono) 
Max Output Power X Channels @ Load  1.9W x 1 @ 4 Ohm  Voltage  Supply  2.7 V ~ 5 V 
Operating Temperature  40°C ~ 85°C  Board Type  Fully Populated 
Utilized Ic / Part  LM48510  Lead Free Status / RoHS Status  Contains lead / RoHS noncompliant 
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The MBR05XX series of diodes are designed to handle a
maximum average current of 0.5A. For applications exceed
ing 0.5A average but less than 1A, a Microsemi UPS5817 can
be used.
DUTY CYCLE
The maximum duty cycle of the boost converter determines
the maximum boost ratio of outputtoinput voltage that the
converter can attain in continuous mode of operation. The
duty cycle for a given boost application is defined as:
Duty Cycle = V
+ V
 V
/ V
1
DIODE
DD
This applies for continuous mode operation.
INDUCTANCE VALUE
The inductor is the largest sized component and usually the
most costly. “How small can the inductor be?” The answer is
not simple and involves tradeoffs in performance. Larger in
ductors mean less 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 switch
ing cycle is:
E = L/2 X (lp)2
Where lp is the peak inductor current. An important point to
observe is that the LM48510 will limit its switch current based
on peak current. This means that since lp(max) is fixed, in
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 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 in
ductor current to drop to zero during the cycle. It should be
noted that all boost converters shift over to discontinuous op
eration as the output load is reduced far enough, but a larger
inductor stays “continuous” over a wider load current range.
To better understand these tradeoffs, a typical application
circuit (5V to 12V boost with a 10µH inductor) will be analyzed.
We will assume:
V
= 5V, V
= 12V, V
IN
OUT
DIODE
Since the frequency is 0.6MHz (nominal), the period is ap
proximately 1.66µs. The duty cycle will be 62.5%, which
means the ONtime of the switch is 1.04µs. It should be noted
that when the switch is ON, the voltage across the inductor is
approximately 4.5V. Using the equation:
V = L (di/dt)
We can then calculate the di/dt rate of the inductor which is
found to be 0.17 A/µs during the ONtime. Using these facts,
we can then show what the inductor current will look like dur
ing operation:
www.national.com
+ V
 V
1
DIODE
SW
During the 1.04µs ONtime, the inductor current ramps up
0.176A and ramps down an equal amount during the OFF
time. This is defined as the inductor “ripple current”. A similar
analysis can be performed on any boost converter, to make
sure the ripple current is reasonable and continuous opera
tion will be maintained 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:
Where "DC" is the duty cycle of the application. The switch
current can be found by:
Inductor ripple current is dependent on inductance, duty cy
cle, input voltage and frequency:
= 0.5V, V
= 0.5V
SW
combining all terms, we can develop an expression which al
lows the maximum available load current to be calculated:
I
LOAD
The equation shown to calculate maximum load current takes
into account the losses in the inductor or turnOFF switching
losses of the FET and diode.
DESIGN PARAMETERS V
The value of the FET ON voltage (referred to as V
tions 4 thru 7) is dependent on load current. A good approxi
mation can be obtained by multiplying the R
times the average inductor current.
12
FIGURE 2. 10μH Inductor Current
5V  12V Boost (LM48510)
)
AMP
I
= I
(AVG) x (1  DC)
LOAD
IND
I
= I
(AVG) + 1/2 (I
)
SW
IND
RIPPLE
I
= DC x (V
V
) / (f x L)
RIPPLE
IN
SW
(max) = (1–DC)x(I
(max)–DC(V
V
SW
IN
SW
AND I
SW
SW
DS(ON)
20123248
(7)
(8)
(9)
))/2fL
(10)
in equa
SW
of the FET
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