MIC21LV32 datasheet(24/50 Pages) MICROCHIP | 36V Dual Phase, Advanced COT Buck Controller Stackable for Multiphase Operation

Part No.	MIC21LV32

Description	36V Dual Phase, Advanced COT Buck Controller Stackable for Multiphase Operation

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MIC21LV32

DS20006513A-page 24

 2021 Microchip Technology Inc.

duty cycle and inductor value. When the high-side

MOSFET is turned off and 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 blanking time of 150 ns for

seven consecutive cycles. This causes the MIC21LV32

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 MIC21LV32 senses the

low-side MOSFET current.

When the MIC21LV32 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 MIC21LV32 initiates the soft start after the

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

FIGURE 4-8:

MIC21LV32 Current Limit

Threshold Relationship to Output Current.

When multiple MIC21LV32 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

MIC21LV32

current

limit

needs

to

be

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

of

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:

Blanking Time

( 150 ns)

Current Limit

Threshold

SW

Soft Start

Sequence

Starts Here

Hiccup Time-out

( 2 ms)

Output

Voltage

Low-Side

MOSFET

Current

High-Side

MOSFET

Current

ON state

OFF

state

1

2

3

4

5

7

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 4-14,

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.

Where:

ILIM = Desired Current Limit per Phase

VILIM = Programmable Voltage at ILIM Pin

ILIM =

0.3 – 0.25 × VILIM

RDSON

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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