MIC2132 datasheet(23/48 Pages) MICROCHIP

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DS20006654B-page 23

MIC2132

the low-side MOSFET is turned on, as shown as

Case #2 in Figure 4-8, the current through the low-side

MOSFET is higher than the current limit after the blank-

ing time of 150 ns for seven consecutive cycles. This

causes the MIC2132 to enter the current limit protection.

As shown in Figure 4-8, the inductor valley current is

higher than the current limit threshold as the MIC2132

senses the low-side MOSFET current.

When the MIC2132 enters current limit protection, both

the high-side and 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 for each phase until it falls down to zero. The

MIC2132 initiates the soft start after the hiccup

time-out, as shown in Figure 4-8.

FIGURE 4-8:

MIC2132 Current Limit

Threshold Relationship to Output Current.

When multiple MIC2132 devices are connected in

parallel to support higher load current, all ILIM pins are

connected together and one resistor is connected from

ILIM to AGND to set the current limit. Only the host

device forces the 9.6 µA current from the ILIM pin. The

secondaries do not force any current, they just read

VILIM. All phases share the same VILIM value under

stackable phase configuration.

The

MIC2132

current

limit

needs

temperature-insensitive when precise sense resistors

or RDSON of the low-side MOSFETs are used.

Since the RDSON resistance increases to about two

times, from +25°C to +125°C, an external NTC resistor

is required to program the current limit in this case. In

case regular precise sense resistors are used, no NTC

resistance is needed.

To realize a positive temperature coefficient from the

negative

temperature

coefficient

the

NTC

resistance, the current limit per phase was internally

generated, as shown in Equation 4-13.

EQUATION 4-13:

From Equation 4-13, one can derive the VILIM value

through Equation 4-14.

EQUATION 4-14:

To program the target VILIM voltage, Equation 4-15 is

used.

EQUATION 4-15:

EXAMPLE 4-1:

CALCULATION OF RILIM

FOR BOTTOM MOSFET

RDSON CURRENT SENSING

As shown in Example 4-1, the sizing of current limit per

phase needs to be verified over temperature in order to

make sure Equation 4-13 and Equation 4-14 work

correctly, because it is necessary to always have:

EQUATION 4-16:

For linearization and fitting the temperature coefficient

of the bottom MOSFET RDSON, a network from the ILIM

pin to AGND is used with an NTC resistor, as shown in

Figure 4-9.

Blanking ime

(≈150 ns)

Current Limit

Threshold

Soft Start

equence

tarts

ere

Hiccup Time out

(≈2 ms)

Output

Voltage

Low

MOSFET

urrent

High

MOSFET

urrent

ON state

OFF

state

Short on

Output

7 onsecutive urrent mit vents

Case#1

Case#2

Where:

ILIM = Desired Current Limit per Phase

VILIM = Programmable Voltage at ILIM Pin

ILIM =

0.3V – 0.25 × VILIM

RDSON

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

+25°C; using Equation 4-14,

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 increases 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.

VILIM = 1.2V – 4 × RDSON × ILIM

Where:

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

ILIM Pin

RILIM = Current Limit Threshold Voltage

Programming Resistance

VILIM = ICL × RILIM

1.2V > 4 × RDSON × ILIM

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