MIC2194_05 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – 400kHz SO-8 Buck Control IC

MIC2194
The MIC2194 controller is broken down into several func-
tions.
• Control loop
• PWM operation
• Current mode control
• Current limit
• Reference, enable and UVLO
• MOSFET gate drive
• Oscillator
Control Loop
PWM Control Loop
The MIC2194 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.
V_IN
Switching
Converter
V_OUT
Switch
Driver
I_INDUCTOR
Voltage
Divider
V_ERROR
V_REF
I_INDUCTOR
V_ERROR
t_ON
t_PER
D = t_ON/t_PER
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.
Micrel
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_SENSE
=
MIN
_
CURRENT _ SENSE
I_{OUT _ MAX}
_
THRESHOLD
The maximum output current is:
I_{OUT _ MAX}
=
MAX
_
CURRENT _ SENSE
R_SENSE
_
THRESHOLD
The current sense pins VIN (pin 8) and CSL (pin 4) are noise
sensitive due to the low signal level, high input impedance
and input ripple voltage. The PCB traces should be short and
routed close to each other. A 0.1µF 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 5) is greater than its
undervoltage threshold.
• The voltage on the enable pin (pin 3) is greater
than the enable UVLO threshold.
The enable pin (pin 3) has two threshold levels, allowing the
MIC2194 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 MIC2194 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 output drivers are inhibited from
switching. The OUTP pin is in a high state. Raising the enable
voltage above the standby threshold enables the output
driver. 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.
April 2005
7
MIC2194