ML1565 datasheet(12/21 Pages) MINILOGIC

12 / 21 page

P12/21

Rev. C, Sep 2005

ML1565

Detailed Description (3)

Maximum Duty Cycle

The ML1565 auxiliary PWM controllers have a guaranteed

maximum duty cycle of 80%. That is to say that all

controllers can achieve at least 80% and typically reach

85%. In boost designs that employ continuous current, the

maximum duty cycle limits the boost ratio such that:

1-VIN / VOUT ≤ 80%

With discontinuous inductor current, no such limit exists

for the input/output ratio since the inductor has time to fully

discharge before the next cycle begins.

Fault Protection

The ML1565 has robust fault and overload protection.

After power-up, the device is set to detect an out-of

regulation state that could be caused by an overload or

short, If any DC-to-DC converter channel (step-up.

Step-down, or any of the auxiliary controllers) remains

faulted for 100,000 clock cycles, then ALL outputs latch off

until the step-up DC-to-DC converter is reinitialized by the

ONSU pin, or by cycling of input power. The fault-detection

circuitry for any channel is disabled during its initial turn-on

soft-start sequence.

Note that output of the step-up, or that of any auxiliary

channel set up in boost configuration, does not fall to 0V

during shutdown or fault. This is due to the current path

from the battery to the output that remains even when the

channel is off. This path exists through the boost inductor

and the synchronous rectifier body diode. An auxiliary

boost channel falls to the input voltage minus the rectifier

drop during fault and shutdown. OUTSU falls to the input

voltage minus the synchronous rectifier body diode drop

during shutdown, and also during fault if the input voltage

exceeds 2.5V. If the input voltage is less than 2.5V,

OUTSU remains at 2.5V due to operation of the startup

oscillator, but can source only limited current.

Reference

The ML1565 has internal 1.250V reference. Connect a

0.1µF ceramic bypass capacitor from REF to GND within

0.2in (5mm) of the REF pin. REF can source up to 200µA

and is enabled whenever ONSD is high and OUTSD is

above 2.5V. If the application requires that REF be loaded

beyond 200µA, it may be buffered with a unity-gain

amplifier or op amp.

Oscillator

All ML1565 DC-to-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

setting is 100kHz to 1MHz.

The oscillator uses a comparator, a 300ns one-shot, and

an internal N-FET 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 Rosc Cosc product. The comparator output switches

high when the capacitor voltage reaches VREF(1.25V). In

turn, the one-shot activates the internal MOSFET switch to

discharge the capacitor within a 300ns interval, and the

cycle repeats. Note that the oscillation frequency changes

as the main output voltage ramps upward following startup.

The oscillation frequency is constant once the main output

is in regulation.

Figure 4 Master Oscillator

ML1565

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