MIC22205 Datasheet, PDF (16/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
The MIC22205 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,
depending on the size, cost, and efficiency, and still
keep
the
LC
natural
frequency
⎜⎜⎝⎛
1
2×π×
L×C
⎟⎟⎠⎞
less than 26 kHz to ensure stability. The minimum
recommended inductor value is 0.47µH and minimum
recommended output capacitor value is 22µF. The
trade-off with 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 1:
CÆ
LÈ
22µF ̶ 47µF 47µF ̶ 100µF 100µF ̶ 470µF
0.47µH
1µH
0* ̶ 10pF
0^† ̶ 15pF
22pF
15 ̶ 22pF
33pF
33pF
2.2µH
15 ̶ 33pF
33 ̶ 47pF
* VOUT > 1.2V, ^† VOUT > 1V
100 ̶ 220pF
Note: Compensation values for various output voltages and inductor
values refer to Table 3.
Table 1. Compensation Capacitor Selection
Feedback
The MIC22205 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
⎜⎛ ^VOUT
⎜⎝ _VREF
− 1_{⎟⎟⎠⎞}
MIC22205
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 (R1) 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.
Enable/Delay (EN/DLY) Pin
Enable/Delay (EN/DLY) sources 1µA out of the IC to
allow a startup delay to be implemented. The delay time
is the time it takes 1µA to charge C_EN/DLY to 1.25V.
Therefore:
t _EN/DLY
=
1.24 × C_EN/DLY
1×10 ^{− 6}
CF Capacitor
Adding a capacitor to this pin can adjust the switching
frequency from 800kHz to 4MHz. CF sources 400µA out
of the IC to charge the CF capacitor in order to set up
the switching frequency. The switch period is the time it
takes 400µA to charge CF to 1.0V. Therefore:
CF Capacitor
56pF
68pF
82pF
100pF
150pF
180pF
220pF
270pF
330pF
390pF
470pF
Frequency
4.4MHz
4MHz
3.4MHz
2.8MHz
2.1MHz
1.7MHz
1.4MHz
1.2MHz
1.1MHz
1.05MHz
1MHz
Table 2. CF vs. Frequency
It is necessary to connect the CF capacitor very close
between the CF pin and signal ground.
August 2011
16
M9999-082511-A