MIC22405 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 4A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
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
below.
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
100 ̶ 220pF
* VOUT > 1.2V, ^† VOUT > 1V
Table 1. Compensation Capacitor Selection
Note: Compensation values for various output voltages and
inductor values refer to table 3.
Feedback
The MIC22405 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_{⎟⎟⎠⎞}
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 simply the time it takes 1µA to charge C_EN/DLY to
1.25V. Therefore:
t _EN/DLY
= ^{1.24 × CEN/DLY}
1×10 ^{− 6}
CF Capacitor
Adding a capacitor to this pin can adjust switching
frequency from 800kHz to 4MHz. CF sources 400µA out
of the IC to charge the CF capacitor to set up the
switching frequency. The switch period is simply the time
it takes 400µA to charge CF to 1.0V. Therefore:
MIC22405
CF Capacitor Frequency
56pF
4.4MHz
68pF
4MHz
82pF
3.4MHz
100pF
2.8MHz
150pF
2.1MHz
180pF
1.7MHz
220pF
1.4MHz
270pF
1.2MHz
330pF
1.1MHz
390pF
1.05MHz
470pF
1MHz
Table 2. CF vs. Frequency
It is necessary to connect the CF capacitor between the
CF pin and signal ground.
300kHz to 800kHz Operation
The frequency range can be lowered by adding an
additional resistor (R_CF) in parallel with the CF capacitor.
This reduces the amount of current used to charge the
capacitor, reducing the frequency. The following
equation can be used to for frequencies between
800kHz to 300kHz.:
− R_CF
× C_CF
× ln_{⎜⎜⎝⎛}1+
1.0V
400μ0 × R_CF
⎟⎟⎠⎞
=
t
R_CF > 2.9KΩ
RC Pin (Soft-Start)
The RC pin provides a trimmed 1µA current source/sink
for accurate ramp-up (soft-start). This allows the
MIC22405 to be used in systems requiring voltage
tracking or ratio-metric voltage tracking at startup.
There are two ways of using the RC pin:
1. Externally driven from a voltage source
2. Externally attached capacitor sets output ramp-
up/down rate
In the first case, driving RC with a voltage from 0V to
V_REF will program the output voltage between 0 and
100% of the nominal set voltage as shown in figure 3.
In the second case, the external capacitor sets the ramp-
up and ramp-down time of the output voltage. The time
is given by
t _RAMP
=
0.7 × C_RC
1×10 ^{− 6}
Where t_RAMP is the time from 0 to 100% nominal output
voltage.
During start-up, a light load condition (I_OUT < 1.25A) can
lead to negative inductor current. Under these
June 2011
16
M9999-061511-A