MIC22400_10 Datasheet, PDF (14/28 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
MIC22400
Efficiency
vs. Inductance
94 _4.7µH
92
90
88
86
84
1µH
82
80
78
V_IN = 3.3V
76
0 0.2 0.4 0.6 0.8 1.0 1.2
OUTPUT CURRENT (A)
Figure 4. Efficiency vs. Inductance
Compensation
The MIC22400 has a combination of internal and
external stability compensation to simplify the circuit for
small, high efficiency designs. In such designs, voltage
mode conversion is often the optimum solution. Voltage
mode is achieved by creating an internal 1MHz ramp
signal and using the output of the error amplifier to
modulate the pulse width of the switch node, thereby
maintaining output voltage regulation. With a typical gain
bandwidth of 100 − 200kHz, the MIC22400 is capable of
extremely fast transient responses.
The MIC22400 is designed to be stable with a typical
application using a 1µH inductor and a 47µF ceramic
(X5R) output capacitor. These values can be varied
dependant upon the tradeoff between size, cost and
efficiency, keeping the LC natural frequency
(
1
) ideally less than 26 kHz to ensure
2⋅π ⋅ L⋅C
stability can be achieved. The minimum recommended
inductor value is 0.47µH and minimum recommended
output capacitor value is 22µF. The tradeoff between
changing these values is that with a larger inductor,
there is a reduced peak-to-peak current which yields a
greater efficiency at lighter loads. A larger output
capacitor will improve transient response by providing a
larger hold up reservoir of energy to the output.
The integration of one pole-zero pair within the control
loop greatly simplifies compensation. The optimum
values for C_COMP (in series with a 20k resistor) are shown
in Table 2.
CÆ 22-47µF
LÈ
0.47µH
1µH
0*-10pF
0^†-15pF
2.2µH
15-33pF
* VOUT > 1.2V, ^† VOUT > 1V
47µF-
100µF
22pF
15-22pF
33-47pF
100µF-
470µF
33pF
33pF
100-220pF
Table 2. Compensation Capacitor Selection
Note: For compensation values for various output
voltages and inductor values refer to Table 4.
Feedback
The MIC22400 provides a feedback pin to adjust the
output voltage to the desired level. This pin connects
internally to an error amplifier. The error amplifier then
compares the voltage at the feedback to the internal
0.7V reference voltage and adjusts the output voltage to
maintain regulation. The resistor divider network for a
desired V_OUT is given by:
R2 = ^R1
⎜⎜⎝⎛
V_OUT
V_REF
− 1_{⎟⎟⎠⎞}
where V_REF is 0.7V and V_OUT is the desired output
voltage. A 10kΩ or lower resistor value from the output
to the feedback is recommended since large feedback
resistor values increase the impedance at the feedback
pin, making the feedback node more susceptible to
noise pick-up. A small capacitor (50pF – 100pF) across
the lower resistor can reduce noise pick-up by providing
a low impedance path to ground.
PWM Operation
The MIC22400 is a voltage mode, pulse width
modulation (PWM) controller. By controlling the ratio of
on-to-off time, or duty cycle, a regulated DC output
voltage is achieved. As load or supply voltage changes,
so does the duty cycle to maintain a constant output
voltage. In cases where the input supply runs into a
dropout condition, the MIC22400 will run at 100% duty
cycle.
The MIC22400 provides constant switching from 800kHz
to 4MHz with synchronous internal MOSFETs. The
internal MOSFETs include a 60mΩ high-side P-Channel
MOSFET from the input supply to the switch pin and a
30mΩ N-Channel MOSFET from the switch pin-to-
ground. Since the low-side N-Channel MOSFET
provides the current during the off cycle, a freewheeling
Schottky diode from the switch node-to-ground is not
required.
PWM control provides fixed-frequency operation. By
maintaining a constant switching frequency, predictable
fundamental and harmonic frequencies are achieved.
Other methods of regulation, such as burst and skip
modes, have frequency spectrums that change with load
that can interfere with sensitive communication
equipment.
December 2010
14
M9999-120310-F