MIC2133 datasheet(29/50 Pages) MICROCHIP

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DS20006653B-page 29

MIC2133

Figure 4-17 and Figure 4-18 show how the AVP design

window of ±Ɛ can be used to reduce the amount of the

output capacitor necessary to sustain the load

transient. Alternatively, the AVP can be used to improve

the error of the load transient if it is decided to keep the

same output capacitor.

The DROOP pin is an analog output that provides a

voltage proportional to the output current in CCM,

according to the equation below.

EQUATION 4-23:

Because the current-sensing range is ±120 mV, the

output voltage range of VDROOP is 0V to 0.9V.

The part of the schematic to implement the AVP for a

5V output is shown in Figure 4-19. The underlying

assumption is that the current sense is done using

sense resistors independent of temperature.

The sizing starts with the conditions: VDROOP = 0V for

IOUT = 0A and VDROOP = 600 mV for IOUT = IOUT(MAX).

Depending on the voltage drop on the sense resistors at

IOUT(MAX), the DROOP pin can have a value different than

600 mV, assuming that VDROOP(IOUTMAX) = 600 mV.

Step 1: Sizing the resistors for getting (1 + Ɛ)

⋅VOUT at

ILOAD = 0A. Due to VDROOP = 0V, we have the equation

below:

EQUATION 4-24:

As a first approximation, consider choosing RFBB2

small enough so that RFBB2||RDROOP ≈ RFBB2, and we

size RFBB1, RFBB2 and RFBT to get the correct 5.00V

injection and stability.

Step 2: Sizing the resistors to have the trip of

VOUT *2⋅Ɛ from IOUT = 0A to IOUT = IOUT(MAX), or from

VDROOP = 0V to VDROOP = 600 mV.

Then,

EQUATION 4-25:

If RDROOP >> RFBB2, Equation 4-25 can be simplified

as the following equation.

EQUATION 4-26:

EQUATION 4-27:

Step 3: The RFBB1 is slightly adjusted to get

VOUT⋅(1 + Ɛ) for IOUT = 0A. The result is in the figure

below.

FIGURE 4-19:

AVP Implementation for 5V

Output with 2% AVP Range for DROOP Pin

Range 0V to 600 mV.

Equation 4-26 and Equation 4-27 can also have the

exact solution; the main difficulty being to find standard

resistors of 0.1% to respect the initial positioning of +Ɛ

for IOUT = 0A and the 2⋅Ɛ move down for IOUT(MAX).

The example above was based on temperature-

independent current sensing using sense resistors. In

case the bottom FET is used, the VDROOP is defined as:

EQUATION 4-28:

Considering the sensing current range of 120 mV, the

operating maximum voltage value is:

EQUATION 4-29:

VDROOP

VCSH 1.2V

–

RSENSE IL



Where:

VCSH = Voltage at CSH Pin in CCM

RSENSE = Current Sensing Resistance

IL = Inductor Current per Phase

VOUT

RFBT

RFBB1 RFBB2\\RDROOP

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



 V

REF





V

FBS

VREF

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

VDROOP MAX



VREF

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

RFBB2

RFBB2 RDROOP

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

RFBT

RFBT RFBB1 RFBB2\\RDROOP

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





VDROOP MAX



VREF

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

RFBB2

RFBB2 RDROOP

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



RFBT

RFBT RFBB1 RFBB2

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



RDROOP

RFBB2



------

VDROOP MAX



VREF

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

RFBT

RFBT RFBB1 RFBB2

-------------------------------------------------------------------- 1

–











MIC2133

FBS

DROOP

VOUT

RINJ

CINJ

RIP_INJ

GFB

RDROOP

CFF

RFBT

RFBB1

RFBB2

RBIAS

VDROOP

VCSH 1.2V 8 RDSON LS

 IL



–

Where:

RDSON(LS) = Low-Side MOSFET Turn-On Resistance

VDROOP MAXOP



120 mV 8

0.96V



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