MAX1534 Datasheet, PDF (13/16 Pages) Maxim Integrated Products – High-Efficiency, Triple-Output, Keep-Alive Power Supply for Notebook Computers

High-Efficiency, Triple-Output, Keep-Alive
Power Supply for Notebook Computers
V_OUT3
50mV/div
AC-COUPLED
10V
V_LX
10V/div
0
1A
I_LX
500mA/div
0
4µs/div
V_IN = 12V, I_OUT3 = 300mA
Figure 3. Normal Buck Operation
voltage. For optimum performance, select an inductor
with the lowest possible DC resistance that fits in the
allotted dimensions. Some recommended component
manufacturers are listed in Table 2.
Maximum Buck Output Current
The MAX1534’s buck converter’s maximum output cur-
rent is limited by the peak inductor current. For the typi-
cal application, the maximum output current is
approximately:
I_OUT3(MAX) = 1/2 I_{LX (PEAK)(MIN)}
For low-input voltages, the maximum on-time can be
reached and the load current is limited by:
I_OUT3 = 1/2 (V_IN - V_OUT3) ✕ 10µs / L
Note that any current provided by the linear regulators
comes from the buck regulator and subtracts from the
maximum current that the buck provides for other loads.
Buck Output Capacitor Selection
Choose the output capacitor to service the maximum
load current with acceptable voltage ripple. The output
ripple has two components: variations in the charge
stored in the output capacitor with each LX pulse, and
the voltage drop across the capacitor’s equivalent
series resistance (ESR) caused by the current into and
out of the capacitor:
V_RIPPLE ≅ V_RIPPLE(ESR) + V_RIPPLE(C)
The output voltage ripple as a consequence of the ESR
and output capacitance is:
V_RIPPLE(ESR) = ESR ✕ I_PEAK
V_OUT1
R_TOP1
R_BOT1
V_OUT2
R_TOP2
R_BOT2
OUT1
PRESET
IN
FB1
MAX1534
LX
OUT2
FB3
FB2
GND
V_IN = +7V TO +24V
R_TOP3
V_OUT3
R_BOT3
Figure 4. Adjustable Output Voltages
V_RIPPLE(C)
=
L × (I_PEAK
2C_OUT3
− I_OUT3)²
× V_OUT3


V_IN
V_IN
− V_OUT3


where I_PEAK is the peak inductor current (see Inductor
Selection). The worst-case ripple occurs at no load.
These equations are suitable for initial capacitor selec-
tion, but final values should be set by testing a proto-
type or evaluation circuit. As a general rule, a smaller
amount of charge delivered in each pulse results in
less output ripple. Since the amount of charge deliv-
ered in each oscillator pulse is determined by the
inductor value and input voltage, the voltage ripple
increases with larger inductance, and as the input volt-
age decreases. See Table 1 for recommended capaci-
tor values and Table 2 for recommended component
manufacturers.
Buck Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple-current
requirement (I_RMS) imposed by the switching current
defined by the following equation:
I_RMS
=
I_OUT3 × V_OUT3
V_IN


4
3 
×
V_IN
V_OUT3
−1
For most applications, nontantalum chemistries (ceram-
ic, aluminum, polymer, or OSCON) are preferred due to
their robustness to high inrush currents typical of sys-
tems with low-impedance battery inputs. Choose an
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