MIC2186 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Low Voltage PWM Control IC

MIC2186
The reverse voltage requirement of the diode is:
For the MIC2186, Schottky diodes are recommended when
they can be used. They have a lower forward voltage drop
than ultra-fast rectifier diodes, which lowers power dissipa-
tion and improves efficiency. They also do not have a recov-
ery time mechanism, which results in less ringing and noise
when the diode turns off. If the output voltage of the circuit
prevents the use of a Schottky diode, then only ultra-fast
recovery diodes should be used. Slower diodes will dissipate
more power in both the MOSFET and the diode. The will also
cause excessive ringing and noise when the diode turns off.
Reference, Enable and UVLO Circuits
The output drivers are enabled when the following conditions
are satisfied:
• The Vdd voltage (pin 10) is greater than its
undervoltage threshold.
• The voltage on the enable pin is greater than the
enable UVLO threshold.
The internal bias circuitry generates a 1.245V bandgap
reference for the voltage error amplifier and a 3V Vdd voltage
for the internal supply bus. The reference voltage in the
MIC2186 is buffered and brought out to pin 8. The Vref pin
must be bypassed to GND (pin 4) with a 0.1uf capacitor. The
Vdd pin must be decoupled to ground with a 1uf ceramic
capacitor.
The enable pin (pin 7) has two threshold levels, allowing the
MIC2186 to shut down in a micro-current mode, or turn off
output switching in standby mode. Below 0.9V, the device is
forced into a micro-power shutdown. If the enable pin is
between 0.9V and 1.5V the output gate drive is disabled but
the internal circuitry is powered on and the soft start pin
voltage is forced low. There is typically 135mV of hysteresis
below the 1.5V threshold to insure the part does not oscillate
on and off due to ripple voltage on the input. Raising the
enable voltage above the UVLO threshold of 1.5V enables
the output drivers and allows the soft start capacitor to
charge. The enable pin may be pulled up to VinA.
Oscillator & Sync
The internal oscillator is self-contained and requires no
external components. The HiDC and f/2 pins allow the user
to select from three different switching frequencies and two
maximum duty cycles. The chart in Table 1 shows the four
combinations that can be programmed along with the typical
minimum and maximum duty cycles.
Micrel
F/2 pin HiDC
Level Level
Switching
Frequency
Maximum
Duty Cycle
Typical
Minimum
Duty Cycle
T_OFF in
Skip Mode
0
1
400kHz
85%
7%
1µs
1
1
200kHz
85%
6%
2µs
0
0
200kHz
50%
4%
1µs
1
0
100kHz
50%
3%
2µs
Table 1
Minimum duty cycle becomes important in a boost converter
as the input voltage approaches the output voltage. At lower
duty cycles, the input voltage can be closer to the output
voltage without the output rising out of regulation.
A frequency foldback mode is enabled if the voltage on the
feedback pin (pin 6) is less than 0.3V. In frequency foldback
the oscillator frequency is reduced by approximately a factor
of 4. For the 400kHz setting, the oscillator runs at 100khz in
frequency foldback. For a 200kHz setting the oscillator runs
at approximately 50kHz and for a 100kHz setting, the oscil-
lator runs at approximately 25kHz.
The SYNC input (pin 11) allows the MIC2186 to synchronize
with an external CMOS or TTL clock signal. Depending on the
setting of the HiDC pin,the output frequency is either equal to
or 1/2 of the sync input frequency. If the HiDC level is low, the
output switching frequency is half the sync frequency. If the
HiDC level is high, the output switching frequency is equal to
the sync frequency.
The rising edge of the sync signal generates a reset signal in
the oscillator, which turns off the high-side gate drive output.
The low-side drive is turned on, restarting the switching cycle.
The sync signal is inhibited when the controller operates in
skip mode or frequency foldback. The sync signal frequency
must be greater than the maximum specified free running
frequency of the MIC2186. If the synchronizing frequency is
lower, double pulsing of the gate drive outputs will occur.
When not used, the sync pin must be connected to ground.
Table 2 shows the minimum recommended sync frequencies
for the different combinations of f/2 and HiDC inputs.
Figure 8a shows the timing between the external sync signal
(trace 2) and the low-side drive (trace 1) for a high level on the
HiDC pin. Figure 8b shows the timing between the external
sync signal (trace 2) and the low-side drive (trace 1) for a low
level on the HiDC pin. The sync frequency is twice the output
switching frequency.
MIC2186
F/2 pin
Level
0
1
0
1
HiDC Self Oscillating
Level Frequency
1
400kHz
1
200kHz
0
200kHz
0
100kHz
Minimum
Recommended
Sync Frequency
480kHz
250kHz
480kHz
250kHz
Sync Input Frequency
f_S=output switching frequency
f_SYNC=sync input frequency
f =f
S SYNC
f_S = f_SYNC
f_S = ¹/₂ f_SYNC
f_S = ¹/₂ f_SYNC
Table 2
12
July 2002