MIC22600_11 Datasheet, PDF (14/32 Pages) Micrel Semiconductor – 1MHz, 6A Integrated Switch 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.
L
22-47µF
0.47µH
1µH
0*-10pF
0^†-15pF
2.2µH
15-33pF
* VOUT > 1.2V, ^† VOUT > 1V
C
47µF-
100µF
22pF
15-22pF
33-47pF
100µF-
470µF
33pF
33pF
100-220pF
Table1. Compensation Capacitor Selection
Feedback
The MIC22600 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 MIC22600 is a voltage mode, pulse width
modulation (PWM) controller. By controlling the 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
MIC22600 will run at 100% duty cycle.
The MIC22600 provides constant switching at 1MHz with
synchronous internal MOSFETs. The internal 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.
MIC22600
PWM control provides fixed-frequency operation. By
maintaining a constant switching frequency, predictable
fundamental and harmonic frequencies are achieved.
Sequencing and Tracking
The MIC22600 provides additional pins to provide
up/down sequencing and tracking capability for
connecting multiple voltage regulators together.
EN/DLY Pin
The EN 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 EN 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 SV_IN. When FB falls below 90% of nominal, POR
is asserted low immediately. However, if EN 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_TP+1.24V)-1.24V) is crossed (V_TP is the internal
voltage clamp, V_TP-=0.9V), 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 MIC22600
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
June 2011
14
M9999-062411-D