July 2002

13

MIC2186

MIC2186

Micrel

MIC2186 Sync

Conditions:

HiDC = HIGH

F/2 = LOW

Gat Drive Output

(Pin 14)

Sync Input

(Pin 11)

Figure 8a.

MIC2186 Sync

Conditions:

HiDC = LOW

F/2 = LOW

Gat Drive Output

(Pin 14)

Sync Input

(Pin 11)

Figure 8b.

The maximum recommended output switching frequency is

600kHz. Synchronizing to higher frequencies may be pos-

sible, however there are some concerns. As the switching

frequency is increased, the switching period decreases. The

minimum on-time in the MIC2186 becomes a greater part of

the total switching period. This may prevent proper operation

as Vin approaches Vout and may also minimize the effective-

ness of the current limit circuitry. The maximum duty cycle

decreases as the sync frequency is increased. Figure 9

shows the relationship between the minimum/maximum duty

cycle and frequency.

40

45

50

55

60

65

70

75

80

85

250 300 350 400 450 500 550 600

SYNC FREQUENCY (kHz)

Max. Duty Cycle

vs. Frequency

HiDC=L

F/2=H

HiDC=L

F/2=L

HiDC=H

F/2=L

HiDC=H

F/2=H

Figure 9.

Soft start

Soft start reduces the power supply input surge current at

start up by limiting the output voltage risetime. Input surge

current occurs when the boost converter charges up the

output capacitance. Slowing the output risetime lowers the

input surge current. Soft start may also be used for power

supply sequencing. The soft start cannot control the initial

surge in current in a boost converter when Vin is applied. This

surge 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 capaci-

tors.

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

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

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

start capacitor low.

* When the Vdd voltage drops below its UVLO

threshold

* When the enable pin drops below its 1.5V 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 MIC2186 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 Figure 10 show 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 Start

0V

Figure 10.

Voltage Setting Components

The MIC2186 requires two resistors to set the output voltage

as shown in figure 11