MIC45205 datasheet(15/31 Pages) MICREL

15 / 31 page

Micrel, Inc.

MIC45205

Figure 4. MIC45205-1 Control Loop Timing

(Discontinuous Mode)

During discontinuous mode, the bias current of most

circuits is substantially reduced. As a result, the total

power supply current during discontinuous mode is only

about 350µA, allowing the MIC45205-1 to achieve high

efficiency in light load applications.

Soft-Start

Soft-start reduces the input power supply surge current at

startup by controlling the output voltage rise time. The

input surge appears while the output capacitor is charged

up.

The MIC45205 implements an internal digital soft-start by

making the 0.8V reference voltage VREF ramp from 0 to

100% in about 5ms with 9.7mV steps. Therefore, the

output voltage is controlled to increase slowly by a stair-

case VFB ramp. Once the soft-start cycle ends, the related

circuitry is disabled to reduce current consumption.

PVDD must be powered up at the same time or after VIN

to make the soft-start function correctly.

Current Limit

The MIC45205 uses the RDS(ON) of the low-side MOSFET

and external resistor connected from ILIM pin to SW

node to set the current limit.

Figure 5. MIC45205 Current-Limiting Circuit

In each switching cycle of the MIC45205, the inductor

current is sensed by monitoring the low-side MOSFET in

the OFF period. The sensed voltage VILIM is compared

with the power ground (PGND) after a blanking time of

150ns. In this way the drop voltage over the resistor R15

(VCL) is compared with the drop over the bottom FET

generating the short current limit. The small capacitor

(C15) connected from ILIM pin to PGND filters the

switching node ringing during the off-time allowing a

better

short-limit

measurement.

The

time

constant

created by R15 and C6 should be much less than the

minimum off time.

The VCL drop allows programming of short limit through

the value of the resistor (R15). If the absolute value of the

voltage drop on the bottom FET becomes greater than

VCL, and the VILIM falls below PGND, an overcurrent is

triggered causing the IC to enter hiccup mode. The

hiccup sequence including the soft-start reduces the

stress on the switching FETs and protects the load and

supply for severe short conditions.

The short-circuit current limit can be programmed by

using Equation 3.

( )

)

(

CLIM

)

(

R15

∆

−

Eq. 3

Where:

ICLIM = Desired current limit

RDS(ON) = On-resistance of low-side power MOSFET,

mΩ typically.

VCL = Current-limit threshold (typical absolute value is

14mV per the Electrical Characteristics table).

ICL = Current-limit source current (typical value is 70µA,

per the Electrical Characteristics table).

April 15, 2014

Revision 1.0

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