LM48511SQBD National Semiconductor, LM48511SQBD Datasheet - Page 16

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LM48511SQBD

Manufacturer Part Number
LM48511SQBD
Description
BOARD EVAL BOOST C LM48511
Manufacturer
National Semiconductor
Series
Boomer®r
Datasheets

Specifications of LM48511SQBD

Amplifier Type
Class D
Output Type
1-Channel (Mono)
Max Output Power X Channels @ Load
6.7W x 1 @ 4 Ohm
Voltage - Supply
3 V ~ 5.5 V
Operating Temperature
-40°C ~ 85°C
Board Type
Fully Populated
Utilized Ic / Part
LM48511
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
www.national.com
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 (C2). A 0.1uF soft-
start capacitor is recommended for a typical application.
The following table shows the relationship between C
up time and surge current.
V
SELECTING DIODE (D1)
Use a Schottkey diode, as shown in Figure 1. A 30V diode
such as the DFLS230LH from Diodes Incorporated is recom-
mended. 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:
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 LM48511 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.
INDUCTOR SUPPLIES
The recommended inductor for the LM48511 is the
IHLP-2525CZ-01 from Vishay Dale. When selecting an in-
ductor, the continuous current rating must be high enough to
avoid saturation at peak currents. A suitable core type must
be used to minimize switching losses, and DCR losses must
0.22
0.47
DD
(μF)
C
0.1
SS
= 5V, PV
Duty Cycle = (PV1+V
Boost Set-up Time
1
= 7.8V (continuous mode)
(ms)
10.5
21.7
5.1
E = L/2 x I
D
-V
DD
P
2
)/(PV1+V
SS
Input Surge Current
) value presents a trade
D
-V
(mA)
330
255
220
SW
)
P
fixed, in-
SS
start-
(5)
(6)
16
be considered when selecting the current rating. Use shielded
inductors in systems that are susceptible to RF interference.
SETTING THE REGULATOR OUTPUT VOLTAGE (PV1)
The output voltage of the regulator is set through one of two
external resistive voltage-dividers (R3 in combination with ei-
ther R1 or R2) connected to FB (Figure 1). The resistor, R4
is only for compensation purposes and does not affect the
regulator output voltage. The regulator output voltage is set
by the following equation:
Where V
R2). To simplify resistor selection:
A value of approximately 25.5kΩ is recommended for R3.
The quiescent current of the boost regulator is directly related
to the difference between its input and output voltages, the
larger the difference, the higher the quiescent current. For
improved power consumption the following regulator input/
output voltage combinations are recommended:
The values of PV1 are for continuous mode operation.
For feedback path selection, see Regulator Feedback Select
section.
DISCONTINUOUS/CONTINUOUS OPERATION
The LM48511 regulator features two different switching
modes. Under light load conditions, the regulator operates in
a variable frequency, discontinuous, pulse skipping mode,
that improves light load efficiency by minimizing losses due
to MOSFET gate charge. Under heavy loads, the LM48511
regulator automatically switches to a continuous, fixed fre-
quency PWM mode, improving load regulation. In discontin-
uous mode, the regulator output voltage is typically 400mV
higher than the expected (calculated) voltage in continuous
mode.
I
CONVERTER
Although the LM48511 regulator is internally compensated,
an external feed-forward capacitor, (C1) may be required for
stability (Figure 1). The compensation capacitor places a zero
in regulator loop response. The recommended frequency of
the zero (f
In addition to C1, a compensation resistor, R4 is required to
cancel the zero contributed by the ESR of the regulator output
capacitor. Calculate the zero frequency of the output capaci-
tor by:
V
SW
DD
3.0
3.6
FEED-FORWARD COMPENSATION FOR BOOST
5
(V) PV1 (V) R3(kΩ) R
FB
Z
) is 22.2kHz. The value of C1 is given by:
is 1.23V, and R
4.8
7.1
7.8
R
LS
PV1 = V
= (R3V
f
CO
C1 =1 / 2
=1 / 2
25.5
25.5
25.5
FB
FB
[1+R3/R
) / (PV1–V
LS
π
π
R
is the low side resistor (R1 or
R3f
CO
LS
9.31
5.35
4.87
C
(kΩ) P
Z
O
LS
]
FB
)
OUT
into 8Ω (W)
2.5
1
3
(10)
(7)
(8)
(9)

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