Datasheet

LM48511
SNAS416G –JULY 2007–REVISED MAY 2013 www.ti.com
INDUCTOR SUPPLIES
The recommended inductor for the LM48511 is the IHLP-2525CZ-01 from Vishay Dale. When selecting an
inductor, 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 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 either 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:
PV1 = V
FB
[1+R3/R
LS
]
Where
• V
FB
is 1.23V, and R
LS
is the low side resistor (R1 or R2) (7)
To simplify resistor selection:
R
LS
= (R3V
FB
) / (PV1–V
FB
) (8)
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:
V
DD
(V) PV1 (V) R3(kΩ) R
LS
(kΩ) P
OUT
into 8Ω (W)
3.0 4.8 25.5 9.31 1
3.6 7.1 25.5 5.35 2.5
5 7.8 25.5 4.87 3
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 frequency PWM mode, improving load regulation. In discontinuous mode, the
regulator output voltage is typically 400mV higher than the expected (calculated) voltage in continuous mode.
I
SW
FEED-FORWARD COMPENSATION FOR BOOST 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
Z
) is 22.2kHz. The value of C1 is given by:
C1 = 1 / 2πR3f
Z
(9)
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 capacitor by:
f
CO
= 1 / 2πR
CO
C
O
where
• R
CO
is the ESR of the output capacitor (10)
The value of R
FB3
is given by:
R4 = 1 / 2πf
CO
C1 (11)
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