MIC2570 Micrel, Inc.
MIC2570 6 August 2007
Functional Description
The MIC2570 switch-mode power supply (SMPS) is a gated
oscillator architecture designed to operate from an input
voltage as low as 1.3V 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 compara-
tor 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
OUT
experiences
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 ap-
proximately 3× larger than the input voltage. Other output
voltages are also easily generated but at a small cost in ef-
ficiency. 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
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.
5V
0V
5V
0mA
I
PEAK
V
IN
Supply
Voltage
Inductor
Current
Output
Voltage
Time
Figure 1. Typical Boost Regulator Waveforms
Synchronization
The SYNC pin is used to synchronize the MIC2570 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 MIC2570 is internally set with a resis-
tor. It functions by modifying the oscillator duty cycle and
frequency. 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
OUT
+ V
DIODE
– V
IN
V
OUT
+ V
DIODE
– V
SAT
Time
Inductor Current
Current “ratchet”
without current limit
Current Limit
Threshold
Continuous
Current
Discontinuous
Current
Figure 2. Current Limit Behavior
