MIC2133 datasheet(25/50 Pages) MICROCHIP

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DS20006653B-page 25

MIC2133

In each switching cycle of both phases of the MIC2133

converter, the inductor valley current is sensed by mon-

itoring the VDS voltage across the low-side MOSFET

during the off period. There is a 150 ns (typical) blanking

period before each current sense signal considered for

protection. The blanking period improves noise immu-

nity. If the valley low-side MOSFET current is greater

than the current limit threshold current for seven consec-

utive cycles in either phase, then the MIC2133 turns off

the high-side MOSFET of both phases and a soft start

sequence is triggered after the hiccup timer has expired.

This mode of operation, called Hiccup mode, and its pur-

pose is to protect the downstream load in case of a hard

short. The figure below illustrates the MIC2133 opera-

tion during overload conditions. When the load current is

increased gradually, the inductor current also increases,

as shown in the figure below. When the load current is

around the current limit threshold, the high-side and the

low-side MOSFET current can be higher than the cur-

rent limit, as highlighted in the figure below as Case#1.

In Case#1, even though the low-side MOSFET instanta-

neous current exceeds the current limit threshold for

some duration, the low-side MOSFET current is lower

than the current limit at the end of the blanking time of

150 ns. This causes the MIC2133 to not enter the cur-

rent limit protection and initiate the next high-side MOS-

FET turn-on cycle. After the high-side MOSFET is turned

on, the current ramps up to a value that is determined by

the operating duty cycle and inductor value. When the

high-side MOSFET is turned off and the low-side MOS-

FET is turned on, as shown as Case#2 in the figure

below, the current through the low-side MOSFET is

higher than the current limit for seven consecutive

cycles. This causes the MIC2133 to enter the current

limit protection.

As shown in the figure below, the inductor valley current

is higher than the current limit threshold as the MIC2133

senses the low-side MOSFET current.

When the MIC2133 enters current limit protection, both

the high-side and the low-side MOSFETs are turned off

for both phases for a hiccup time-out of 2 ms. The

inductor current flows through the body diode of the

low-side MOSFET until it falls down to zero. The

MIC2133 initiates the soft start after the hiccup

time-out, as shown in the figure below.

FIGURE 4-11:

MIC2133 Current-Limit

Threshold Relationship to Output Current.

The MIC2133 current limit needs to be temperature-

insensitive when precise sense resistors or RDSON of

the low-side MOSFETs are used.

The RDSON resistance increases to about two times

from 25°C to 125°C; therefore, an external NTC resis-

tor is used to program the current limit in this case. In

case regular precise sense resistors are used, no NTC

resistance is needed.

To achieve a positive temperature coefficient from the

negative

temperature

coefficient

the

NTC

resistance, the current limit per phase was internally

generated, as shown in the equation below.

EQUATION 4-16:

From Equation 4-16, one can derive the VILIM value

through the equation below:

EQUATION 4-17:

To program the target VILIM voltage, the equation below

is used.

EQUATION 4-18:

EXAMPLE 4-2:

CALCULATION OF RILIM

FOR BOTTOM MOSFET

RDSON CURRENT SENSING

Blanking time

( 150ns)

Current Limit

Threshold

Soft Start

sequence

starts here

Hiccup Timeout

( 2ms)

Output

Voltage

Low side

MOSFET

current

High side

MOSFET

current

ON state

OFF

state

Short on

Output

7 consecutive current limit events

Case#1

Case#2

• For ILIM = 10A per phase, RDSON = 10 mΩ at

25°C; using Equation 4-17,

VILIM = 1.2V – 4 * 10 mΩ * 10A = 1.2V – 0.4V = 0.8V

at 25°C.

• To get 0.8V on the ILIM pin with a 9.6 µA

constant-current source, we need a

programming equivalent resistance of

RILIM = 0.8V/9.6 µA = 83.3 kΩ at 25°C.

• If the temperature is increased to 125°C, then

RDSON at 125°C = 20 mΩ at the same 10A limit.

• Therefore, VILIM = 1.2V – 4 * 20 mΩ * 10A =

1.2V – 0.8V = 0.4V at 125°C. Then,

RILIM = 0.4V/9.6 µA = 41.7 kΩ at 125°C.

ILIM

0.3 0.25 VILIM



–

RDSON

--------------------------------------------

Where:

ILIM = Desired Current Limit per Phase

VILIM = Programmable Voltage at ILIM Pin

VILIM

1.2V 4

–



DSON

ILIM



VILIM

ICL RILIM



Where:

ICL = 9.6 µA (typical) Constant-Current Source at

ILIM Pin

RILIM = Current Limit Threshold Voltage

Programming Resistance

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