Rainbow Electronics MAX1585 - Manual (Page 19)

iary converters and cannot be activated until PVSU is in
regulation. For automatic startup, connect ON_ to PVSU
or a logic level greater than 1.6V.
Design Procedure
Setting the Switching Frequency
Choose a switching frequency to optimize external
component size or circuit efficiency for the particular
application. Typically, switching frequencies between
400kHz and 500kHz offer a good balance between
component size and circuit efficiency—higher frequen-
cies generally allow smaller components, and lower fre-
quencies give better conversion efficiency. The
switching frequency is set with an external timing resistor
(R
OSC ) and capacitor (C OSC ). At the beginning of a
cycle, the timing capacitor charges through the resistor
until it reaches V
REF . The charge time, t 1 , is as follows:
t 1 = -R OSC x C OSC x l n (1 - 1.25 / V PVSU )
The capacitor voltage then decays to zero over time t 2
= 150ns. The oscillator frequency is as follows:
f OSC = 1 / (t 1 + t 2 )
f
OSC
can be set from 100kHz to 1MHz. Choose C
OSC
between 22pF and 470pF. Determine R OSC :
R OSC = (150ns - 1 / f OSC ) / (C OSC x l n [1 - 1.25
V PVSU ])
See the Typical Operating Characteristics section for
f
OSC
vs. R
OSC
using different values of C
OSC
.
Setting Output Voltages
The MAX1584/MAX1585 step-up and step-down con-
verters and the AUX1 controllers have resistor-
adjustable output voltages. When setting the voltage for
all channels except AUX2 on the MAX1585, connect a
resistive voltage-divider from the output voltage to the
corresponding FB_ input. The FB_ input bias current is
less than 100nA, so choose the low-side (FB_-to-GND)
resistor (R
L ) to be 100k
Ω or less. Then calculate the
high-side (output-to-FB_) resistor (R H ):
R H = R L [(V OUT / 1.25) - 1]
AUX2 is an inverter on the MAX1585, so the FB2
threshold on the MAX1585 is 0V. To set the MAX1585
AUX2 negative output voltage, connect a resistive volt-
age-divider from the negative output to the FB2 input,
and then to REF. The FB2 input bias current is less than
100nA, so choose the REF-side (FB2-to-REF) resistor
(R
REF ) to be 100k Ω or less. Then calculate the top-side
(negative output-to-FB2) resistor:
R TOP = R REF (-V OUT(AUX2) / 1.25)
General Filter-Capacitor Selection
The input capacitor in a DC-DC converter reduces cur-
rent peaks drawn from the battery or other input power
source and reduces switching noise in the controller.
The impedance of the input capacitor at the switching
frequency should be less than that of the input source
so high-frequency switching currents do not pass
through the input source.
The output capacitor keeps output ripple small and
ensures control-loop stability. The output capacitor
must also have low impedance at the switching fre-
quency. Ceramic, polymer, and tantalum capacitors
are suitable, with ceramic exhibiting the lowest ESR
and high-frequency impedance.
Output ripple with a ceramic output capacitor is
approximately:
V RIPPLE = I L(PEAK) [1 / (2 π x f
OSC x C OUT )]
If the capacitor has significant ESR, the output ripple
component due to capacitor ESR is:
V
RIPPLE(ESR)
= I
L(PEAK)
x ESR
Output capacitor specifics are also discussed in each
converter’s Compensation section.
Step-Up Component Selection
The external components required for the step-up are
an inductor, an input and output filter capacitor, and a
compensation RC.
The inductor is typically selected to operate with contin-
uous current for best efficiency. An exception might be
if the step-up ratio, (V
OUT / V IN ), is greater than 1 / (1 -
D MAX ), where D MAX is the maximum PWM duty factor
of 80%.
When using the step-up channel to boost from a low
input voltage, loaded startup is aided by connecting a
Schottky diode from the battery to PVSU. See the
Minimum Startup Voltage vs. Load Current graph in the
Typical Operating Characteristics section.
Step-Up Inductor
In most step-up designs, a reasonable inductor value
(L
IDEAL ) can be derived from the following equation,
which sets continuous peak-to-peak inductor current at
half the DC inductor current:
L IDEAL = [2V IN(MAX) x D(1 - D)] / (I OUT x f OSC )
where D is the duty factor given by:
D = 1 - (V IN / V OUT )
Given L IDEAL , the consistent peak-to-peak inductor cur-
rent is 0.5 x I OUT / (1 - D). The peak inductor current is
as follows:
MAX1584/MAX1585
5-Channel Slim DSC Power Supplies
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