MIC2238 Datasheet, PDF (11/16 Pages) Micrel Semiconductor – Dual Synchronous 800mA/800mA Step-Down DC/DC Regulator

Micrel, Inc.
external capacitor at the PGOOD pin will cause the
PGOOD pin voltage to rise from low to high in around
390µs.
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
designs using the adjustable output voltage option. To
reduce battery current draw, a 100kΩ feedback resistor
is recommended for use from the output to the FB pin
(R1). Also, a feedforward capacitor should be
MIC2238
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. Typically, a 22pF small ceramic
capacitor is recommended.
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.
July 2007
11
M9999-071307