October 2005

13

MIC2185

MIC2185

Micrel, Inc.

Soft Start

Soft Start reduces the power supply input surge current at

start up by limiting the output voltage rise time. Input surge

currentoccurswhentheboostconverterchargesuptheoutput

capacitance. Slowing the output rise time lowers the input

surge current. Soft Start may also be used for power supply

sequencing. The soft start cannot control the initial surge of

current is caused by the output capacitance charging up to

the input voltage. The current flows from the input through

the inductor and output diode to the output capacitors.

The soft start voltage is applied directly to the PWM compara-

tor. A 5µA internal current source is used to charge up the

soft start capacitor. Either of 2 UVLO conditions will pull the

soft start capacitor low.

threshold

• When the Enable pin drops below the UVLO

threshold

The part switches at a low duty cycle when the soft start pin

voltage zero. As the soft start voltage rises from 0V to 0.7V,

the duty cycle increases from the minimum duty cycle to

the operating duty cycle. The oscillator runs at the foldback

frequency until the feedback voltage rises above 0.3V. In

a boost converter the output voltage is equal to the input

voltage before the MIC2185 starts switching. If the ratio of

Vout/Vin is low, the voltage on the feedback pin will already

be greater than 0.3V and the converter begin switching at

the selected operating frequency.

The risetime of the output is dependent on the soft start

capacitor, output capacitance, input and output voltage and

load current. The scope photo in Figure10 shows the output

voltage and the soft start pin voltage at startup. The output

voltage is initially at the input voltage less a diode drop. After

the converter is enabled the output slowly rises due to the

minimum duty cycle of the controller. As the soft start voltage

increases, the output voltage rises in a controlled fashion until

the output voltage reaches the regulated value.

Soft StartWaveform

TIME (2ms/div)

VSS

1V/div

VOUT

2V/div

0V

Figure 10 Soft Start

Voltage Setting Components

The MIC2185 requires two resistors to set the output voltage

as shown in Figure 11.

Pin

6

Voltage

Amplifier

1.245V

MIC2185

R1

R2

Figure 11

The output voltage is determined by the equation below.

V

V

1 R1

R2

O

REF

=

× +









where:

Lower values of resistance are preferred to prevent noise

value for R1 is 10kΩ.

Decoupling Capacitor Selection

A 1µF decoupling capacitor is used to stabilize the internal

regulator and minimize noise on the VDD pin. Placement

of this capacitor is critical to the proper operation of the

MIC2185. It must be next to the VDD and signal ground

pins. The capacitor should be a good quality ceramic. Incor-

rect placement of the VDD decoupling capacitor will cause

jitter and/or oscillations in the switching waveform as well as

variations in the overcurrent limit.

A minimum 0.1µF ceramic capacitor is required to decouple

connected directly between pin 10 (VDD) and pin 5 (SGND).

A 0.1µF capacitor is required to decouple VREF. It should

be located near the VREF pin.

Efficiency calculation and considerations

Efficiency is the ratio of output power to input power. The

difference is dissipated as heat in the boost converter. The

significant contributors at light output loads are:

current required to switch the external MOSFETs

• Core losses in the inductor

To maximize efficiency at light loads:

• Use a low gate charge MOSFET or use the

smallest MOSFET, which is still adequate for the

maximum output current.

• Allow the MIC2185 to run in skip mode at lower

currents. If running in PWM mode, set the fre-

quency to 200kHz.