MIC45404 datasheet(20/32 Pages) MICROCHIP

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MIC45404

DS20005478A-page 20

 2015 Microchip Technology Inc.

The programmed VIN UVLO threshold, VIN_RISE, is

given by:

EQUATION 5-2:

To desensitize the VIN UVLO threshold against varia-

tions of the pull-up current, EN_I, it is recommended to

run the R1 –R2 voltage divider at a significantly higher

current level than the EN_I current.

The corresponding VIN UVLO hysteresis, VIN_HYS, is

calculated as follows:

EQUATION 5-3:

Similar calculations also apply to the internal LDO

activation threshold.

5.2

Setting the Switching Frequency

The

MIC45404

switching

frequency

can

programmed using FREQ, as shown in Table 5-1.

The switching frequency setting is not arbitrary, but it

needs to be adjusted according to the particular output

voltage selection due to peak-to-peak inductor

ripple requirements. This is illustrated in Section 5.5

“Permissible MIC45404 Settings Combinations”

5.3

Setting the Output Voltage

The MIC45404 output voltage can be programmed by

setting pins, VOSET0 and VOSET1, as shown in

Table 5-2.

To achieve accurate output voltage regulation, the

OUTSNS pin (internal feedback divider top terminal)

should be Kelvin-connected as close as possible to the

point of regulation top terminal. Since both the internal

reference and the internal feedback divider’s bottom

terminal refer to AGND, it is important to minimize

voltage drops between the AGND and the point of

regulation return terminal.

5.4

Setting the Current Limit

The MIC45404’s valley-mode current limit on the

low-side MOSFET can be programmed by means of

ILIM as shown in Table 5-3.

Note that the programmed current limit values act as

pulse-by-pulse, current limit thresholds on the valley

inductor current. If the inductor current has not decayed

below the threshold at the time the PWM requires a new

on time, the high-side MOSFET turn-on is either delayed,

until the valley current recovers below the threshold, or

skipped. Each time the high-side MOSFET turn-on is

skipped, a 4-bit up-down counter is incremented. When

the counter reaches the configuration ‘1111’, a hiccup

sequence is invoked in order to reduce power dissipation

under prolonged short-circuit conditions.

The highest current limit setting (6.8A) is intended to

comfortably accommodate a 5A application. Ensure

that the value of the operating junction temperature

does not exceed the maximum rating in high output

power applications.

TABLE 5-1:

SWITCHING FREQUENCY

SETTINGS

FREQ Pin Setting

Frequency

High Z (open)

400 kHz

(GND)

565 kHz

(VDDA)

790 kHz

IN_RISE

EN_R

------





EN_I



–



Where:

EN_R = 1.21V

EN_I = 2 µA

R1 and R2 = External resistors

IN_HYS

150 mV

------







TABLE 5-2:

OUTPUT VOLTAGE SETTINGS

VOSET1

VOSET0

Output Voltage

(GND)

3.3V

(GND)

(VDDA)

2.5V (2.49V)

(VDDA)

(GND)

1.8V

(VDDA)

(VDDA)1.5V

(GND)

High Z (open)

1.2V

High Z (open)

(GND)

1.0V

(VDDA)

High Z (open)

0.9V

High Z (open)

(VDDA)0.8V

High Z (open)

0.7V

TABLE 5-3:

CURRENT LIMIT SETTINGS

ILIM

Valley Current Limit

(Typical Value)

Rated Output

Current

(GND)

4.6 A

(VDDA)

6.2 A

High Z (open)

6.8 A

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