MIC2253_11 Datasheet, PDF (8/13 Pages) Micrel Semiconductor – 3.5A 1MHz High Efficiency Boost Regulator with OVP and Softstart

Micrel, Inc.
Figure 1. Typical Application Circuit
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can
be calculated as follows for a boost regulator:
D = 1- ^VIN
V_OUT
However at light loads, the inductor will completely
discharge before the end of a switching cycle. The current
in the inductor reaches zero before the end of the switching
cycle. This is known as discontinuous conduction mode
(DCM). DCM occurs when:
where
I_OUT
<
V_IN
V_OUT
× ^IPEAK
2
I_PEAK
<
(V_OUT - V_IN
L×f
)
×
⎜⎜⎝⎛
V_IN
V_OUT
⎟⎟⎠⎞
In DCM, the duty cycle is smaller than in continuous
conduction mode. In DCM the duty cycle is given by:
f×
D=
2 × L × I_OUT × (V_OUT − V_IN )
V_IN
The duty cycle required for voltage conversion should be
less than the maximum duty cycle of 90%. Also, in light load
conditions where the input voltage is close to the output
voltage, the minimum duty cycle can cause pulse skipping.
This is due to the energy stored in the inductor causing the
output to slightly overshoot the regulated output voltage.
During the next cycle, the error amplifier detects the output
as being high and skips the following pulse. This effect can
be reduced by increasing the minimum load or by
increasing the inductor value. Increasing the inductor value
also reduces the peak current. Minimum duty cycle is
typically 10%.
MIC2253
Over-Voltage Protection (OVP)
The MIC2253 provides a fixed 5.6V overvoltage
protection. The overvoltage functionality will clamp the
output voltage to a safe level in the event that a fault
condition causes the output voltage to increase beyond
control. To ensure the highest level of protection, the
MIC2253 OVP pin will shut the switch off when an
overvoltage condition is detected, saving itself, the
output capacitor, and downstream devices from damage.
Two external resistors can be used to change the OVP
from the range of 6V to 30V. Be careful not to exceed
the 30V rating of the switch. The OVP feature may be
disabled by grounding the OVP pin.
The OVP pin is connected internally to a reference
voltage via a voltage divider circuit. For a 5.6V OVP
setting, connect the OVP pin directly to the output
voltage as shown in Figure 1. To increase the OVP
voltage above 5.6V, an external parallel resistor network
can be configured, as shown in Figure 2, with the
following equation:
V_OVP
=
1.245 ×
67k × (R1+ R2)
15k × R2
Figure 2. Adjustable OVP Circuit
Note:
1. The maximum value of R2 is 30kΩ.
Soft Start Functionality
The soft start time is dependant up on both C_SS and the
comp capacitor values. C_COMP is fixed for stable
operation (typically 10nF); therefore, if any increases in
soft start are desired, this should be done using the C_SS
capacitor. The approximate total startup time is given by:
T_SS = 1ms + 85k × C_SS
April 2011
8
M9999-042011-C