MIC2182 Datasheet, PDF (13/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
Switching from PWM to Skip Mode
The current sense amplifier in Figure 3 monitors the average
voltage across the current-sense resistor. The controller will
switch from PWM to skip mode when the average voltage
across the current-sense resistor drops below approximately
12mV. This is shown in Figure 7b. The average output current
at this transition level for is calculated below.
I_{OUT(skipmode)}
=
0.012
R_CS
where:
0.012 = threshold voltage of the internal comparator
R_CS = current-sense resistor value
Switching from Skip to PWM Mode
The frequency of occurrence of the skip-mode current pulses
increase as the output current increases until the hysteretic
duty cycle reaches 100% (continuous pulses). Increasing the
current past this point will cause the output voltage will drop.
The low limit comparator senses the output voltage when it
drops below 2% of the set output and automatically switches
the converter to PWM mode.
The inductor current in skip mode is a triangular wave shape
a minimum value of 0 and a maximum value of 35mV/R_CS
(see Figure 7b). The maximum average output current in skip
mode is the average value of the inductor waveform:
I_{OUT(max skipmode)}
=
0.5
×
35mV
R_CS
The capacitor on the PWM pin (pin 2) is discharged when the
IC transitions from skip to PWM mode. This forces the IC to
remain in PWM mode for a fixed period of time. The added
delay prevents unwanted switching between PWM and skip
mode. The capacitor is charged with a 10uA current source
on pin 2. The threshold on pin 2 is 2.5V. The delay for a typical
1nF capacitor is:
t_delay
=
C_PWM × V_threshold
I_source
=
1nF × 2.5V
10µA
=
250µs
where:
C_PWM = capacitor connected to pin 2
Current Limit
The current-limit circuit operates during PWM mode. The
output current is detected by the voltage drop across the
external current-sense resistor (R_CS in Figure 2.). The cur-
Micrel
rent-limit threshold is 100mV+35mV –25mV. 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_CS
=
75mV
I_OUT(max)
The maximum output current is:
I_OUT(max)
=
135mV
R_CS
The current-sense pins CSH (pin 8) and V_OUT (pin 9) are
noise sensitive due to the low signal level and high input
impedance. The PCB traces should be short and routed close
to each other. A small (1nF to 0.1µF) capacitor across the pins
will attenuate high frequency switching noise.
When the peak inductor current exceeds the current-limit
threshold, the current-limit comparator, in Figure 2, turns off
the high-side MOSFET for the remainder of the cycle. The
output voltage drops as additional load current is pulled from
the converter. When the output voltage reaches approxi-
mately 0.95V, the circuit enters frequency-foldback mode
and the oscillator frequency will drop to 60kHz while maintain-
ing the peak inductor current equal to the nominal 100mV
across the external current-sense resistor. 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 11) is greater than its
undervoltage threshold (typically 4.2V).
• The voltage on the enable pin is greater than the
enable UVLO threshold (typically 2.5V)
The internal bias circuit generates a 1.245V bandgap refer-
ence voltage for the voltage error amplifier and a 5V V_DD
voltage for the gate drive circuit. The reference voltage in the
fixed-output-voltage versions of the MIC2182 is buffered and
brought to pin 7. The V_REF pin should be bypassed to GND
(pin 4) with a 0.1µF capacitor. The adjustable version of the
MIC2182 uses pin 7 for output voltage sensing. A decoupling
capacitor on pin 7 is not used in the adjustable output voltage
version.
Inductor
Current
I_LIM(skip)
0A
Figure 7a. Maximum Skip-Mode-Load Inductor Current
35mV THRESHOLD
ACROSS RCS.
Inductor
Current
I_MIN(PWM)
0A
12mV THRESHOLD
OF AVERAGE VOLTAGE
ACROSS RCS.
Figure 7b. Minimum PWM-Mode-Load Inductor Current for PWM Operation
June 2000
13
MIC2182