MIC2875 datasheet(13/24 Pages) MICROCHIP

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DS20005549A-page 13

MIC2875

5.0

APPLICATION INFORMATION

5.1

General Description

The MIC2875 is a 2 MHz, current-mode, PWM,

synchronous boost converter with an operating input

voltage range of 2.5V to 5.5V. At light load, the

converter enters pulse-skipping mode to maintain high

efficiency over a wide range of load current. The

maximum peak current in the boost switch is limited to

4.8A (typical).

5.2

Bi-Directional Output Disconnect

The power stage of the MIC2875 consists of a NMOS

transistor as the main switch and a PMOS transistor as

the synchronous rectifier. A control circuit turns off the

back gate diode of the PMOS to isolate the output from

the input supply when the chip is disabled (VEN = 0V).

An “always on” maximum supply selector switches the

cathode of the back-gate diode to either the IN or the

OUT (whichever of the two has the higher voltage). As

a result, the output of the MIC2875 is bi-directionally

isolated from the input as long as the device is

disabled. The maximum supply selector and hence the

output disconnect function requires only 0.3V at the IN

pin to operate.

5.3

Minimum Switching Frequency

When the MIC2875 enters the pulse-skipping mode for

more than 20 μs, an internal control circuitry forces the

PMOS to turn on briefly to discharge VOUT to VIN

through the inductor. When the inductor current

reaches a predetermined threshold, the PMOS is

turned off and the NMOS is turned on so that the

inductor current can decrease gradually. Once the

inductor current reaches zero, the NMOS is eventually

turned off. The above cycle repeats if there is no

switching activity for another 20 μs, effectively

maintaining a minimum switching frequency of 45 kHz.

The frequency control circuit is disabled when VOUT is

less than or within 200 mV of VIN. This minimum

switching frequency feature is advantageous for

applications that are sensitive to low-frequency EMI,

such as audio systems.

5.4

Integrated Anti-Ringing Switch

The MIC2875 includes an anti-ringing switch that

eliminates the ringing on the SW node of a

conventional boost converter operating in the

discontinuous conduction mode (DCM). At the end of a

switching cycle during DCM operation, both the NMOS

and PMOS are turned off. The anti-ringing switch in the

MIC2875 clamps the SW pin voltage to IN to dissipate

the remaining energy stored in the inductor and the

parasitic elements of the power switches.

5.5

Automatic Bypass Mode (when

VIN > VOUT)

The MIC2875 automatically operates in bypass mode

when the input voltage is higher than the target output

voltage. In bypass mode, the NMOS is turned off while

the PMOS is fully turned-on to provide a very low

impedance path from IN to OUT.

5.6

Soft-Start

The MIC2875 integrates an internal soft-start circuit to

limit the inrush current during start-up. When the device

is enabled, the PMOS is turned-on slowly to charge the

output capacitor to a voltage close to the input voltage.

Then, the device begins boost switching cycles to

gradually charge up the output voltage to the target

VOUT.

5.7

Output Voltage Programming

The MIC2875 has an adjustable version that allows the

output voltage to be set by an external resistor divider

R2 and R3. The typical feedback voltage is 900 mV, the

recommended maximum and minimum output voltage

is 5.5V and 3.2V, respectively. The current through the

resistor divider should be significantly larger than the

current into the FB pin (typically 0.01 μA). It is

recommended that 0.1% tolerance feedback resistors

must be used and the total resistance of R2 + R3

should be around 1 MΩ. The appropriate R2 and R3

values for the desired output voltage are calculated as

in Equation 5-1:

EQUATION 5-1:

5.8

Current Limit Protection

The MIC2875 has a current limit feature to protect the

part against heavy loading condition. When the current

limit comparator determines that the NMOS switch has

a peak current higher than 4.8A, the NMOS is turned off

and the PMOS is turned on until the next switching

cycle. The overcurrent protection is reset cycle by cycle

OUT

0.9V

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

–

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