MAX1584 Datasheet, PDF(18/29 Page) Maxim Integrated Products

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MAX1584 Datasheet, PDF (18/29 Pages) Maxim Integrated Products – 5-Channel Slim DSC Power Supplies

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5-Channel Slim DSC Power Supplies

VSU

ROSC

OSC

COSC

VREF

(1.25V)

150ns

ONE-SHOT

MAX1584

MAX1585

(PARTIAL)

VBATT

AUX

PWM

PVSU

DL_

+15V

50mA

LCD

MAX1584

MAX1585

FB_

Figure 4. Oscillator Functional Diagram

end of the soft-start interval. If PVSU is dragged below

the input, the overload is supplied by the body diode of

the internal synchronous rectifier or by a Schottky diode

connected from the battery to PVSU. If desired, this

overload current can be interrupted by a P-channel

MOSFET controlled by SCF, as shown in Figure 15.

Reference

The MAX1584/MAX1585 have internal 1.250V refer-

ences. Connect a 0.1ÂµF ceramic bypass capacitor from

REF to GND within 0.2in (5mm) of the REF pin. REF can

source up to 200ÂµA and is enabled when ONSU is high

and PVSU is above 2.5V. The auxiliary controllers and

MAX1801 slave controllers (if connected) each sink up

to 30ÂµA REF current during startup. If the application

requires that REF be loaded beyond 200ÂµA, buffer REF

with a unity-gain amplifier or op amp.

Oscillator

All MAX1584/MAX1585 DC-DC converter channels

employ fixed-frequency PWM operation. The operating

frequency is set by an RC network at the OSC pin. The

range of usable settings is 100kHz to 1MHz. When

MAX1801 slave controllers are added, they operate at

the frequency set by OSC.

The oscillator uses a comparator, a 150ns one-shot,

and an internal NFET switch in conjunction with an

external timing resistor and capacitor (Figure 4). When

the switch is open, the capacitor voltage exponentially

approaches the step-up output voltage from zero with a

time constant given by the product of ROSC and COSC.

The comparator output switches high when the capaci-

tor voltage reaches VREF (1.25V). In turn, the one-shot

NOTE: THIS CIRCUIT CAN OPERATE WITH AUX1 OR

AUX2 ON THE MAX1584, AND WITH AUX1 ON THE MAX1585

Figure 5. +15V LCD Bias with Basic Boost Topology

activates the internal MOSFET switch to discharge the

capacitor within a 150ns interval, and the cycle

repeats. The oscillation frequency changes as the main

output voltage ramps upward following startup. The

oscillation frequency is then constant once the main

output is in regulation.

Low-Voltage Startup Oscillator

The MAX1584/MAX1585 internal control and reference-

voltage circuitry receive power from PVSU and do not

function when PVSU is less than 2.5V. To ensure low-

voltage startup, the step-up employs a low-voltage

startup oscillator that activates at 0.9V if a Schottky rec-

tifier is connected from VBATT to PVSU (1.1V with no

Schottky rectifier). The startup oscillator drives the inter-

nal N-channel MOSFET at LXSU until PVSU reaches

2.5V, at which point voltage control is passed to the

current-mode PWM circuitry.

Once in regulation, the MAX1584/MAX1585 operate

with inputs as low as 0.7V since internal power for the

IC is supplied by PVSU. At low input voltages, the step-

up can have difficulty starting into heavy loads (see the

Minimum Startup Voltage vs. Load Current graph in the

Typical Operating Characteristics section); 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 15).

ON_ Control Inputs

The step-up converter activates with a high input at

ONSU. The step-down and auxiliary DC-DC converters

1, 2, and 3 activate with a high input at ONSD, ON1,

ON2, and ON3, respectively. The step-down and auxil-

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