MAX1566-MAX1567 Datasheet, PDF(27/35 Page) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

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MAX1566-MAX1567 Datasheet, PDF (27/35 Pages) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

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Six-Channel, High-Efficiency, Digital

Camera Power Supplies

up may have difficulty starting into heavy loads (see the

Minimum Startup Voltage vs. Load Current (OUTSU)

graph in the Typical Operating Characteristics); however,

this can be remedied by connecting an external P-

channel load switch driven by SCF so the load is not

connected until the PVSU is in regulation (Figure 17).

Shutdown

The step-up converter is activated with a high input at

ONSU. The main converter (step-up or step-down) is acti-

vated by a high input on ONM. The step-down and auxil-

iary DC-to-DC converters 1, 2, and 3 activate with high

inputs at ONSD, ON1, ON2, and ON3, respectively. The

step-down, main, and AUX_ converters cannot be activat-

ed until PVSU is in regulation. For automatic startup, con-

nect 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 efficiencyâhigher frequen-

cies generally allow smaller components, and lower fre-

quencies give better conversion efficiency. The

switching frequency is set with an external timing resis-

tor (ROSC) and capacitor (COSC). At the beginning of a

cycle, the timing capacitor charges through the resistor

until it reaches VREF. The charge time, t1, is as follows:

t1 = -ROSC x (COSC + Cpar) x ln (1 - 1.25 / VPVSU)

where Cpar (15pF typ) is the parasitic capacitance at

the OSC pin due to internal ESD protection structure

and the die-to-package capacitance.

The internal comparator that compares the capacitor

COSC voltage to the reference has a delay td of 50ns

(typ). The capacitor voltage then decays to zero over

time, t2 = 200ns. The oscillator frequency is as follows:

fOSC = 1 / (t1 + td + t2)

fOSC can be set from 100kHz to 1MHz. Choose COSC

between 22pF and 470pF. Determine ROSC:

ROSC = (200ns + 50ns - 1/ fOSC ) /

([COSC + Cpar] ln[1 - 1.25 / VPVSU])

See the Typical Operating Characteristics for fOSC vs.

ROSC using different values of COSC.

Setting Output Voltages

All MAX1566/MAX1567 output voltages are resistor set.

The FB_ threshold is 1.25V for all channels except for

FB3L (0.2V) on both devices and FB2 (inverter) on the

MAX1567. When setting the voltage for any channel

except the MAX1567 AUX2, connect a resistive volt-

age-divider from the channel output to the correspond-

ing FB_ input and then to GND. The FB_ input bias

current is less than 100nA, so choose the bottom-side

(FB_-to-GND) resistor to be 100kâ¦ or less. Then calcu-

late the top-side (output-to-FB_) resistor:

RTOP = RBOTTOM[(VOUT / 1.25) - 1]

When using AUX3 to drive white LEDs (Figure 7), select

the LED current-setting resistor (R3, Figure 7) using the

following formula:

R3 = 0.2V / ILED

The FB2 threshold on the MAX1567 is 0V. To set the

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

(RREF) to be 100kâ¦ or less. Then calculate the top-side

(output-to-FB2) resistor:

RTOP = RREF(-VOUT(AUX2) / 1.25)

General Filter Capacitor Selection

The input capacitor in a DC-to-DC converter reduces

current peaks drawn from the battery or other input

power source and reduces switching noise in the con-

troller. 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 frequency.

Ceramic, polymer, and tantalum capacitors are suitable,

with ceramic exhibiting the lowest ESR and high-frequen-

cy impedance.

Output ripple with a ceramic output capacitor is

approximately as follows:

VRIPPLE = IL(PEAK)[1 / (2Ï x fOSC x COUT)]

If the capacitor has significant ESR, the output ripple

component due to capacitor ESR is as follows:

VRIPPLE(ESR) = IL(PEAK) x ESR

Output capacitor specifics are also discussed in each

converterâs Compensation section.

Step-Up Component Selection

This section describes component selection for the

step-up, as well as for the main, if SUSD = PV.

The external components required for the step-up are

an inductor, an input and output filter capacitor, and a

compensation RC.

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