MIC2571 Datasheet, PDF (6/11 Pages) Micrel Semiconductor – Single-Cell Switching Regulator Preliminary Information

MIC2571
Micrel
Functional Description
The MIC2571 switch-mode power supply (SMPS) is a gated
5V
oscillator architecture designed to operate from an input
V_IN
voltage as low as 0.9V and provide a high-efficiency fixed or
0V
adjustable regulated output voltage. One advantage of this
architecture is that the output switch is disabled whenever the
I_PEAK
output voltage is above the feedback comparator threshold
thereby greatly reducing quiescent current and improving
0mA
efficiency, especially at low output currents.
Refer to the Block Diagrams for the following discription of
5V
typical gated oscillator boost regulator function.
Time
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
Figure 1. Typical Boost Regulator Waveforms
external resistor divider and compares it to the bandgap _{Synchronization}
reference voltage.
The SYNC pin is used to synchronize the MIC2571 to an
When the voltage at the inverting input of the comparator is _{external oscillator or clock signal. This can reduce system}
below 0.22V, the comparator output is high and the output of _{noise by correlating switching noise with a known system}
the oscillator is allowed to pass through the AND gate to the _{frequency. When not in use, the SYNC pin should be}
output driver and output switch. The output switch then turns _{grounded to prevent spurious circuit operation. A falling edge}
on and off storing energy in the inductor. When the output _{at the SYNC input triggers a one-shot pulse which resets the}
switch is on (low) energy is stored in the inductor; when the _{oscillator. It is possible to use the SYNC pin to generate}
switch is off (high) the stored energy is dumped into the output _{oscillator duty cycles from approximately 20% up to the}
capacitor which causes the output voltage to rise.
nominal duty cycle.
4
When the output voltage is high enough to cause the com- _{Current Limit}
parator output to be low (inverting input voltage is above
0.22V) the AND gate is disabled and the output switch ^{Current limit for the MIC2571 is internally set with a resistor.}
remains off (high). The output switch remains disabled until ^{It functions by modifying the oscillator duty cycle and fre-}
the output voltage falls low enough to cause the comparator ^{quency. When current exceeds 1.2A, the duty cycle is}
output to go high.
reduced (switch on-time is reduced, off-time is unaffected)
and the corresponding frequency is increased. In this way
There is about 6mV of hysteresis built into the comparator to _{less time is available for the inductor current to build up while}
prevent jitter about the switch point. Due to the gain of the _{maintaining the same discharge time. The onset of current}
feedback resistor divider the voltage at V_OUT experiences _{limit is soft rather than abrupt but sufficient to protect the}
about 120mV of hysteresis for a 5V output.
inductor and output switch from damage. Certain combina-
Appications Information
Oscillator Duty Cycle and Frequency
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
The oscillator duty cycle is set to 67% which is optimized to zero and occurs when:
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
duty cycle ≤ ^{VOUT + VDIODE – VIN}
V_OUT + V_DIODE – V_SAT
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_IN or a
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_OUT) is high
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_OUT drops low enough to cause the
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.
Time
Current “ratchet”
without current limit
Current Limit
Threshold
Continuous
Current
Discontinuous
Current
Figure 2. Current Limit Behavior
1997
4-81