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
current occurs when the boost converter charges up the output
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 in a boost converter when V
IN
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 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.
• When the V
DD
voltage drops below its UVLO
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
V
REF
1.245V
MIC2185
R1
R2
Figure 11
The output voltage is determined by the equation below.
V V 1
R1
R2
O
REF
= × +
where:
V
REF
for the MIC2185 is nominally 1.245V.
Lower values of resistance are preferred to prevent noise
from apprearing on the V
FB
pin. A typically recommended
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
the V
IN
pin. The capacitor should be placed near the IC and
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:
• The V
IN
A pin supply current.
• The V
IN
P pin supply current which includes the
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.
