MIC2571

Micrel

MIC2571

6

1997

Functional Description

The MIC2571 switch-mode power supply (SMPS) is a gated

oscillator architecture designed to operate from an input

voltage as low as 0.9V and provide a high-efficiency fixed or

adjustable regulated output voltage. One advantage of this

architecture is that the output switch is disabled whenever the

output voltage is above the feedback comparator threshold

thereby greatly reducing quiescent current and improving

efficiency, especially at low output currents.

Refer to the Block Diagrams for the following discription of

typical gated oscillator boost regulator function.

The bandgap reference provides a constant 0.22V over a

wide range of input voltage and junction temperature. The

comparator senses the output voltage through an internal or

external resistor divider and compares it to the bandgap

reference voltage.

When the voltage at the inverting input of the comparator is

below 0.22V, the comparator output is high and the output of

the oscillator is allowed to pass through the AND gate to the

output driver and output switch. The output switch then turns

on and off storing energy in the inductor. When the output

switch is on (low) energy is stored in the inductor; when the

switch is off (high) the stored energy is dumped into the output

capacitor which causes the output voltage to rise.

When the output voltage is high enough to cause the com-

parator output to be low (inverting input voltage is above

0.22V) the AND gate is disabled and the output switch

remains off (high). The output switch remains disabled until

the output voltage falls low enough to cause the comparator

output to go high.

There is about 6mV of hysteresis built into the comparator to

prevent jitter about the switch point. Due to the gain of the

feedback resistor divider the voltage at V

about 120mV of hysteresis for a 5V output.

Appications Information

Oscillator Duty Cycle and Frequency

The oscillator duty cycle is set to 67% which is optimized to

provide maximum load current for output voltages approxi-

mately 3

× larger than the input voltage. Other output voltages

are also easily generated but at a small cost in efficiency. The

fixed oscillator frequency (options -1 and -2) is set to 20kHz.

Output Waveforms

The voltage waveform seen at the collector of the output

switch (SW pin) is either a continuous value equal to V

switching waveform running at a frequency and duty cycle set

by the oscillator.

The continuous voltage equal to V

IN

happens when the voltage at the output (V

enough to cause the comparator to disable the AND gate. In

this state the output switch is off and no switching of the

inductor occurs. When V

comparator output to change to the high state the output

switch is driven by the oscillator. See Figure 1 for typical

voltage waveforms in a boost application.

5V

0V

5V

0mA

Time

Figure 1. Typical Boost Regulator Waveforms

Synchronization

The SYNC pin is used to synchronize the MIC2571 to an

external oscillator or clock signal. This can reduce system

noise by correlating switching noise with a known system

frequency.

When not in use, the SYNC pin should be

grounded to prevent spurious circuit operation. A falling edge

at the SYNC input triggers a one-shot pulse which resets the

oscillator. It is possible to use the SYNC pin to generate

oscillator duty cycles from approximately 20% up to the

nominal duty cycle.

Current Limit

Current limit for the MIC2571 is internally set with a resistor.

It functions by modifying the oscillator duty cycle and fre-

quency.

When current exceeds 1.2A, the duty cycle is

reduced (switch on-time is reduced, off-time is unaffected)

and the corresponding frequency is increased. In this way

less time is available for the inductor current to build up while

maintaining the same discharge time. The onset of current

limit is soft rather than abrupt but sufficient to protect the

inductor and output switch from damage. Certain combina-

tions of input voltage, output voltage and load current can

cause the inductor to go into a continuous mode of operation.

This is what happens when the inductor current can not fall to

zero and occurs when:

duty cycle

V

+ V

– V

V

+ V

– V

OUT

DIODE

IN

OUT

DIODE

SAT

≤

Time

Current "ratchet"

without current limit

Current limit threshold

Continuous current

Discontinuous current

Figure 2. Current Limit Behavior