MIC2155 datasheet(17/33 Pages) MICREL

17 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

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 over current (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.

The MIC2155/6 current limit circuit restricts the

maximum output current. If the load tries to draw

additional current the output voltage drops until it is no

longer within regulation limits. At this point (75% of

nominal output voltage) a hiccup current mode is

initiated to protect down stream loads from excessive

current during hard short circuits. This helps reduce the

overall power dissipation in the PWM converter

components during a fault.

Figure 14. Overcurrent Sense Waveforms

The MIC2155/6 only senses current across the low side

MOSFET of Channel 1 since both channels operate in

parallel. This means the total output current limit is

approximately twice the calculated current limit.

Current Limit Setting

The current limit circuit responds to the peak inductor

current flowing through the low-side FET. The value of

RCS can be estimated with the “simple” method or can be

more accurately calculated by taking the inductor ripple

current into account.

The Simple Method

Current limit can be quickly estimated with the following

equation:

RCS = IOUT/2 × RDSON(MAX)/180µA.

Where: RDSON is the maximum on-resistance of the low

side FET at the operating junction temperature

Accurate Method

For designs where ripple current is significant when

compared to IOUT or for low duty cycle operation,

calculating the current setting resistor RCS should take

into account that we are sensing the peak inductor

current and that there is a blanking delay of

approximately 100ns.

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