MIC22601 Datasheet, PDF (12/18 Pages) Micrel Semiconductor – 4 MHz, 6A Integrated Switch Synchronous Buck Regulator

Micrel, Inc.
Where V_REF is 0.7V, R1 is the upper resistor, R2 is the
lower resistor 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 decoupling capacitor (50pF – 100pF)
across the lower resistor (R2) can reduce noise pick-up
by providing a low impedance path to the ground.
PWM Operation
The MIC22601 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 MIC22601 will run at 100% duty
cycle.
The MIC22601 provides constant switching at 4MHz with
synchronous internal MOSFETs. The internal 30mΩ
MOSFETs include a high-side P-Channel MOSFET from
the input supply to the switch pin and an 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 MIC22601 provides additional pins to provide
up/down sequencing and tracking capability for
connecting multiple voltage regulators together.
Enable/DLY 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 1^st up,
last down power sequence.
MIC22601
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, Vout 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 ⋅ C_DELAY
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 MIC22601 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 rate of the output voltage. The rate 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.
Tracking & Sequencing examples
There 4 distinct variations which are easily implemented
using the MIC22601. The 2 Sequencing variations are
Delayed and windowed. The 2 tracking variants are ratio
metric and Normal. The following diagrams illustrate
methods for connecting two MIC22601’s to achieve
these requirements.
May 2009
12
M9999-050509-A