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參數(shù)資料
| 型號： | MIC3000BML |
| 廠商： | MICREL INC |
| 元件分類： | 數(shù)字傳輸電路 |
| 英文描述： | SFP Management IC |
| 中文描述： | ATM/SONET/SDH SUPPORT CIRCUIT, QCC24 |
| 封裝： | 4 X 4 MM, MLF-24 |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 63K |
| 代理商： | MIC3000BML |
October 2002
7
MIC2194
MIC2194
Micrel
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.
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 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 0.1
μ
F 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.
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