ML1565 datasheet(11/21 Pages) MINILOGIC

11 / 21 page

P11/21

Rev. C, Sep 2005

ML1565

Detailed Description (2)

● Buck-Boost Operation

The step-down input can be powered from the output of

the step-up. By cascading these two channels, the

step-down output can maintain regulation even as the

battery voltage falls below the step-down output voltage.

This is especially useful when trying to generate 3.3V

from 1-cell Li+ inputs, or 2.5V from 2-cell alkaline or NiMH

inputs, or when designing a power supply that must

operate

from

both

Li+

and

alkaline/NiMH

inputs.

Compound efficiencies of up to 90% can be achieved

when the step-up and step-down are operated in series.

Note that the step-up output supplies both the step-up

load and the step-down input current when the step-down

is powered from the step-up. The step-down input current

reduces the available step up output current for other

loads.

Direct Battery Step-down Operation

The step-down converter can also be operated directly

from the battery as long as the voltage at INSD does not

exceed OUTSU by more than a Schottky diode forward

voltage. When using this connection, connect a Schottky

diode from the battery input to OUTSU to INSD, which

adds a small additional current drain (of approximately

(VOUTSU – VINSD) / 10kΩ from OUTSU when INSD is not

connected directly to OUTSU.

Step-down direct battery operation improves efficiency for

the step-down output (up to 95%), but limits the upper limit

of the output voltage to 200mV less than the minimum

battery voltage. In 1-cell Li+ designs (with a 2.7V min), the

output can be set up to 2.5V. In 2-cell alkaline or

NiMH designs, the output may be limited to 1.5V or 1.8V,

depending on the minimum allowed cell voltage.

The step-down can only be briefly operated in dropout since

the ML1565 fault protection detects the out-of-regulation

condition and activates after 100,000 OSC cycles, or 200ms

at 500kHz. At that point, all ML1565 channels shut down.

Auxiliary DC-to-DC Controllers

The three auxiliary controllers operate as fixed-frequency

voltage-mode PWM controllers. They do not have internal

MOSFETs, so output power is determined by external

components. The controllers regulate output voltage by

modulating the pulse width of the DL_ drive signal to an

external N-channel MOSFET switch.

Figure 3 shows a functional diagram of an AUX controller

channel. A sawtooth oscillator signal at OSC governs timing.

At the start of each cycle, DL_ goes high, turning on the

external N-FET switch. The switch then turns off when the

internally level-shifted sawtooth rises above COMP_ or when

the maximum duty cycle is exceeded. The switch remains off

until the start of the next cycle. A transconductance error

amplifier forms an integrator at COMP_ so that DC high-loop

gain and accuracy can be maintained.

The auxiliary controllers do not start until the step-up

DC-to-DC output is in regulation. If the step-up, step-down or

any of the auxiliary controllers remains faulted for 100,000

OSC cycles, the all ML1565 channels latch off.

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