MIC2128 datasheet(17/32 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005620A-page 17

MIC2128

Figure 4-2 shows operation of the MIC2128 during load

transient. The output voltage drops due to a sudden

increase in load, which results in the error amplifier

output (VgM) falling below VREF. This causes the

comparator to trigger an on-time event. At the end of

the ON time, a minimum OFF time tOFF(MIN) is

generated to charge the bootstrap capacitor. The next

ON time is triggered immediately after the tOFF(MIN) if

the error amplifier output voltage (VgM) is still below

VREF due to the low feedback voltage. This operation

results in higher switching frequency during load

transients. The switching frequency returns to the

nominal set frequency once the output stabilizes at new

load current level. The output recovery time is fast and

the output voltage deviation is small in the MIC2128

converter due to the varying duty cycle and switching

frequency.

FIGURE 4-2:

MIC2128 Load Transient

Response.

Unlike true current-mode control, the MIC2128 uses

the output voltage ripple to trigger an on-time event. In

order to meet the stability requirements, the MIC2128

feedback voltage ripple should be in phase with the

inductor current ripple and large enough to be sensed

by the internal error amplifier. The recommended

feedback voltage ripple is 20 mV~100 mV over the full

input voltage range. If a low-ESR output capacitor is

selected, then the feedback voltage ripple may be too

small to be sensed by the internal error amplifier. Also,

the output voltage ripple and the feedback voltage

ripple are not necessarily in phase with the inductor

current ripple if the ESR of the output capacitor is very

low. For these applications, ripple injection is required

to ensure proper operation. Refer to Section 5.8

“Ripple Injection”

for details about the ripple injection

technique.

4.2

Soft Start (SS)

Soft Start reduces the power supply inrush current at

start-up by controlling the output voltage rise time. The

MIC2128 features SS pin which allows the user to set

the soft-start time by connecting a capacitor from the

SS pin to AGND. An internal current source of 1.3 µA

charges this capacitor and generates a linear voltage

which is used as the reference for the internal error

amplifier during Soft Start. Once the voltage on this SS

capacitor is above the internal fixed reference of 0.6V,

the error amplifier uses the fixed 0.6V as reference

instead of the voltage on the external SS capacitor.

4.3

Current Limit (ILIM)

The MIC2128 uses the low-side MOSFET RDS(ON) to

sense inductor current. In each switching cycle of the

MIC2128 converter, the inductor current is sensed by

monitoring the voltage across the low-side MOSFET

during the OFF period of the switching cycle during

which low-side MOSFET is ON. An internal current

source of 96 µA generates a voltage across the

external current limit setting resistor RCL as show in the

Figure 4-3.

FIGURE 4-3:

MIC2128 Current Limiting

Circuit.

The ILIM pin voltage (VILIM) is the difference of the

voltage across the low-side MOSFET and the voltage

across the resistor (VCL). The sensed voltage VILIM is

compared with the power ground (PGND) after a

blanking time of 150 ns.

If the absolute value of the voltage drop across the

low-side MOSFET is greater than the absolute value of

the voltage across the current setting resistor (VCL), the

MIC2128 triggers the current limit event. Consecutive

VOUT

VFB

VgM

VDH

VREF

toff(MIN)

Full Load

No Load

PGND

VIN

ILIM

Control

Logic

CURRENT LIMIT

DETECTION

RCL

ICL

MIC2128

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