April 2005 9 M9999-042205
MIC2184 Micrel, Inc.
The MIC2184 controller is broken down into 7 functions.
• Control loop
• PWM operation
• Current mode control
• Current limit
• Reference, enable and UVLO
•FreqOut
• MOSFET gate drive
• Oscillator and Sync
• Soft-start
Control Loop
PWM Control Loop
The MIC2184 uses current mode control to regulate the
output voltage. This dual control loop method (illustrated in
Figure 2) senses the output voltage (outer loop) and the
inductor current (inner loop). It uses inductor current and
output voltage to determine the duty cycle of the buck
converter. Sampling the inductor current effectively removes
the inductor from the control loop, which simplifies compen-
sation.
Switching
Converter
Voltage
Divider
V
REF
V
ERROR
V
ERROR
t
ON
t
PER
D = t
ON
/t
PER
I
INDUCTOR
I
INDUCTOR
Switch
Driver
V
OUT
V
IN
Figure 2. Current Mode Control Example
As shown in Figure 1, the inductor current is sensed by
measuring the voltage across the resistor, R
SENSE
. A ramp is
added to the amplified current sense signal to provide slope
compensation, which is required to prevent unstable opera-
tion at duty cycles greater than 50%.
A transconductance amplifier is used for the error amplifier,
which compares an attenuated sample of the output voltage
with a reference voltage. The output of the error amplifier is
the compensation pin (Comp), which is compared to the
current sense waveform in the PWM block. When the current
signal becomes greater than the error signal, the comparator
turns off the high side drive. The COMP pin provides access
to the output of the error amplifier and allows the use of
external components to stabilize the voltage loop.
Current Limit
The output current is detected by the voltage drop across the
external current sense resistor (R
SENSE
in Figure 1.). The
current sense resistor must be sized using the minimum
current limit threshold. The external components must be
designed to withstand the maximum current limit. The current
sense resistor value is calculated by the equation below:
R
MIN CURRENT SENSE THRESHOLD
I
SENSE
OUT MAX
=
___
_
The maximum output current is:
I
MAX CURRENT SENSE THRESHOLD
R
OUT MAX
SENSE
_
___
=
The current sense pins CSH (pin 9) and CSL (pin 8) are noise
sensitive due to the low signal level and high input imped-
ance. The PCB traces should be short and routed close to
each other. A small (1nF) capacitor across the pins will
attenuate high frequency switching noise.
When the peak inductor current exceeds the current limit
threshold, the overcurrent comparator turns off the high side
MOSFET for the remainder of the switching cycle, effectively
decreasing the duty cycle. The output voltage drops as
additional load current is pulled from the converter. When the
voltage at the feedback pin (FB) reaches approximately 0.3V,
the circuit enters frequency foldback mode and the oscillator
frequency will drop to 1/4 of the switching frequency. This
limits the maximum output power delivered to the load under
a short circuit condition.
Reference, Enable and UVLO Circuits
The output drivers are enabled when the following conditions
are satisfied:
• The V
DD
voltage (pin 10) is greater than its
undervoltage threshold.
• The voltage on the enable pin (pin 7) is greater
than the enable UVLO threshold.
The enable pin (pin 7) has two threshold levels, allowing the
MIC2184 to shut down in a low current mode, or turn off output
switching in standby mode. An enable pin voltage lower than
the shutdown threshold turns off all the internal circuitry and
places the MIC2184 in a micropower shutdown mode.
If the enable pin voltage is between the shutdown and
standby thresholds, the internal bias, V
DD
and reference
voltages are turned on. The soft start pin is forced low by an
internal discharge MOSFET. The output drivers are inhibited
from switching. The OUTP pin is in a high state and the OUTN
pin remains in a low state. Raising the enable voltage above
the standby threshold allows the soft start capacitor to charge
and enables the output drivers. The standby threshold is
specified in the electrical characteristics. A resistor divider
can be used with the enable pin to prevent the power supply
from turning on until a specified input voltage is reached. The
circuit in Figure 3 shows how to connect the resistors.
