MIC2155 datasheet(17/35 Pages) MICREL

17 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

FREQUENCY (Hz)

-10

-20

-30

-40

200

160

120

-40

-80

-120

-160

-200

10k

100k

10M

Phase

Magnitude

Figure 11. Remote Sense Amplifier Gain/Phase Plot

A typical remote sense configuration is shown in Figure

12. The output of the remote sense amplifier feeds a

voltage divider (R1, R4), which is connected to the

Channel 1 error amplifier. The divider and compensation

network for the remote sense are the same as for a local

sense configuration. The 10Ω resistors provide an

alternate feedback path if the remote sense connections

are removed or opened. The remote sense connections

should not be shorted or the output voltage will increase

close to VIN. The OVP circuit in the controller will not

protect against this type of fault since the feedback pin

voltage will be 0V.

LOAD

+SENSE

–SENSE

Figure 12. Remote Sense

Setting the Output Voltage

Regardless of whether the remote sensing or local

output voltage sensing is used, the output voltage is set

with voltage divider resistors R1 and R4 (Figure 12). The

equation below is used to calculate VOUT.

⎥⎦

⎤

⎢⎣

⎡ +

REF

OUT

Where:

VREF = 0.7V

Current Limit and Overcurrent Protection

The MIC2155/6 uses the synchronous (low-side)

MOSFETs RDSON to sense an over current condition. The

low-side MOSFET is used because it displays lower

parasitic oscillations after switching then the upper

MOSFET. Additionally, it improves the accuracy and

reduces false tripping at lower voltage outputs and

narrow duty cycles since the off-time increases as duty

cycle decreases.

200µA

MIC2155/6

HSD

CS1

RCS

Current Limit

VIN

–IL × RDSON

LSD

Figure 13. Overcurrent Circuit

Inductor current flows from the lower MOSFET source to

the drain during the off-time. The drain voltage becomes

negative with respect to ground as the inductor current

continues to flow from Source to Drain. This negative

voltage is proportional to instantaneous inductor current

times the MOSFET RDSON. The voltage across the low-

side FET becomes even more negative as the output

current increases. The overcurrent circuit operates by

passing a known fixed current source (200µA) through a

resistor RCS. This sets up an offset voltage (ICS × RCS)

that is compared to the VDS of the low-side FET. When

ISD (Source to Drain current) × IL is equal to this voltage,

the MIC2155’s over current trigger is set, which disables

the next high side gate drive pulse. After missing the

high side pulse, the overcurrent (OC) trigger is reset. If

on the next low-side drive cycle, the current is still too

high i.e. VCS is ≤ 0V, another high side pulse is missed

and so on. This effectively reduces the overall energy

transferred to the output and VOUT starts to fall.

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