MIC2267 Datasheet, PDF (9/19 Pages) Micrel Semiconductor – Input Current Limiting Synchronous Buck Regulator

Micrel Inc.
Functional Diagram
MIC2267
Figure 1. MIC2267 Functional Block Diagram
Functional Description
The MIC2267 is an input current limited, 2A synchronous
buck regulator. The part offers control of the input
average current limit and the time constant of the current
limit response, allowing full control of the input current
profile during load steps and plug in events. The P-
Channel high side switch allows for 100% duty cycle
operation. Optimization of the loop bandwidth can be
achieved using the available connections to the slope
compensation ramp and error amplifier nodes. For logic
control and error flagging, the MIC2267 has an enable
function and a Power Good function.
V_IN
The input supply (V_IN) provides power to the internal
power MOSFETs and the analog control circuitry. The
V_IN operating range is 3V to 5.5V. A minimum bypass
capacitor of 10µF should be placed close to the input
(V_IN) pin and the ground (PGND) pin. Refer to the Layout
Recommendations section for details on placing the
input capacitor.
LIMIT
The current limit as set by the R_LIMIT resistor (Nominally,
I_LIMIT = 75k/R_LIMIT) is converted to the total charge
allowed in one cycle (Q_TOT). So if the current limit is 1A
and the frequency is 1MHz, since Q = I x T, the total
charge allowed Q_TOT = 1µC. If for example, a 1µF
capacitor C_TOT is used to store this charge, since Q = C x
V, the voltage would be 1V.
If a replica of the input current is then integrated and the
resultant input charge per cycle:
Q_IN = ∫_OTI_IN(t).dt
Is stored on a similar 1µF capacitor C_INT, then input
current limit is reached when the voltage on the C_INT
capacitor reaches 1V, i.e. Q_IN ≥ Q_TOT. Once this
condition is satisfied and the FILTER delay time has
elapsed, the P-Channel switch cycle is terminated. At
the beginning of each cycle the integration storage
capacitors are discharged. In the actual circuit the
storage capacitor is in the order of 5pF and the replica
current is scaled down accordingly.
January 2011
9
M9999-011711-A