MIC2185 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
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 capaci-
tors.
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 Start Waveform
VOUT
2V/div
VSS
1V/div
0V
TIME (2ms/div)
Figure 10 Soft Start
Voltage Setting Components
The MIC2185 requires two resistors to set the output voltage
as shown in Figure 11
MIC2185
Micrel
Voltage
Amplifier
V_REF
1.245V
R1
Pin
6 _R2
Figure 11
The output voltage is determined by the equation below.
V_O= V_REF × ^{}1+ _RR21^{}
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. Incorrect place-
ment 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_INA pin supply current.
• The V_INP 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
frequency to 200kHz.
• Use a ferrite material for the inductor core, which
has less core loss than an MPP or iron power
core.
May 2002
13
MIC2185