MIC2238-AAYMLTR Datasheet, PDF (10/16 Pages) Micrel Semiconductor – 2.5MHz Dual Phase PWM Buck Regulator

Micrel, Inc.
time is approximately 1µs per 1pF of capacitance. For
example, a 390pF capacitor at the PGOOD pin will
cause the PGOOD pin voltage to rise from low to high in
around 390µs. A PGOOD capacitor is recommended to
prevent large output voltage transients from triggering
the PGOOD flag unexpectedly.
Figure 1. Power Good Circuit
FB1/FB2
The feedback pin (FB) provides the control path to
control the output. For adjustable versions, a resistor
divider connecting the feedback to the output is used to
adjust the desired output voltage. The output voltage is
calculated as follows:
V_OUT
=
V_REF
×
⎛ _R1
⎝⎜R2
⎞
+ 1_⎠⎟
where V_REF is equal to 0.8V.
A feedforward capacitor is recommended for most
MIC2238
designs using the adjustable output voltage option. To
reduce battery current draw, a 100KΩ feedback resistor
is recommended from the output to the FB pin (R1).
Also, a feedforward capacitor should be connected
between the output and feedback (across R1). The large
resistor value and the parasitic capacitance of the FB pin
can cause a high frequency pole that can reduce the
overall system phase margin. By placing a feedforward
capacitor, these effects can be significantly reduced.
Refer to the Feedback section for recommended
feedforward capacitor values.
SW1/SW2
The switch (SW) pin connects directly to the inductor
and provides the switching current necessary to operate
in PWM mode. Due to the high speed switching on this
pin, the switch node should be routed away from
sensitive nodes.
PGND
Power ground (PGND) is the ground path for the high
current PWM mode. The current loop for the power
ground should be as small as possible and separate
from the Analog ground (AGND) loop. Refer to the layout
considerations for more details.
AGND
Signal ground (AGND) is the ground path for the biasing
and control circuitry. The current loop for the signal
ground should be separate from the Power ground
(PGND) loop. Refer to the layout considerations for more
details.
April 2010
10
M9999-040810-A