MIC22400 Datasheet, PDF (14/19 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
Feedback
The MIC22400 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
⎜⎜⎝⎛
V_OUT
V_REF
− 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 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.
PWM Operation
The MIC22400 is a voltage mode, pulse width
modulation (PWM) controller. By controlling the ratio of
on-to-off time, or duty cycle, a regulated DC output
voltage is achieved. As load or supply voltage changes,
so does the duty cycle to maintain a constant output
voltage. In cases where the input supply runs into a
dropout condition, the MIC22400 will run at 100% duty
cycle.
The MIC22400 provides constant switching from 800kHz
to 4MHz with synchronous internal MOSFETs. The
internal MOSFETs include a 60mΩ high-side P-Channel
MOSFET from the input supply to the switch pin and a
30mΩ N-Channel MOSFET from the switch pin-to-
ground. Since the low-side N-Channel MOSFET
provides the current during the off cycle, a freewheeling
Schottky diode from the switch node-to-ground is not
required.
PWM control provides fixed frequency operation. By
maintaining a constant switching frequency, predictable
fundamental and harmonic frequencies are achieved.
Other methods of regulation, such as burst and skip
modes, have frequency spectrums that change with load
that can interfere with sensitive communication
equipment.
Sequencing and tracking
The MIC22400 provides additional pins to provide
up/down sequencing and tracking capability for
connecting multiple voltage regulators together.
MIC22400
Enable pin
The Enable pin contains a trimmed, 1µA current source
which can be used with a capacitor to implement a fixed
desired delay in some sequenced power systems. The
threshold level for power on is 1.24V with a hysteresis of
20mV.
Delay Pin
The Delay pin also has a 1µA trimmed current source
and a 1µA current sink which acts with an external
capacitor to delay the operation of the Power On Reset
(POR) output. This can be used also in sequencing
outputs in a sequenced system, but with the addition of a
conditional delay between supplies; allowing a first up,
last down power sequence.
After Enable is driven high, V_OUT will start to rise (rate
determined by RC capacitor). As the FB voltage goes
above 90% of its nominal set voltage, Delay begins to
rise as the 1µA source charges the external capacitor.
When the threshold of 1.24V is crossed, POR is
asserted high and Delay continues to charge to a
voltage V_DD. When FB falls below 90% of nominal, POR
is asserted low immediately. However, if enable is driven
low, POR will fall immediately to the low state and Delay
will begin to fall as the external capacitor is discharged
by the 1µA current sink. When the threshold of V_DD-
1.24V is crossed, V_OUT will begin to fall at a rate
determined by the RC capacitor. As the voltage change
in both cases is 1.24V, both rising and falling delays are
matched at
T _POR
= ^{1.24 ⋅ CDLY}
1.10 ^−6
RC pin
The RC pin provides a trimmed 1µA current source/sink
similar to the Delay Pin for accurate ramp up (soft start)
and ramp down control. This allows the MIC22400 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.
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.
February 2008
14
M9999-022108-A