MIC22705_11 Datasheet, PDF (16/29 Pages) Micrel Semiconductor – 1MHz, 7A 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
* VOUT > 1.2V, ^† VOUT > 1V
100 ̶ 220pF
Feedback
The MIC22705 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}
MIC22705
RC Pin (Soft-Start)
The RC pin provides a trimmed 1µA current source/sink
for accurate ramp up (soft-start). This allows the
MIC22705 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
conditions, the maximum ramp up time should not
exceed the critical ramp up time period to keep regulator
in continuous mode operation when V_FB reaches 90% of
reference voltage.
Beyond the critical ramp up time, the regulator is in
discontinuous mode which leads to prolonged N-channel
FET conduction, which in turn causes negative inductor
current.
The maximum C_RC value is calculated as follows:
C _RC
<
2.86 × C_OUT ×L ×F_SW
⎜⎜⎝⎛^1−
V_OUT
V_IN
⎟⎟⎠⎞
×10 ^−6
March 2011
16
M9999-033111-A