MIC2196_08 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – 400kHz SO-8 Boost Control IC

Micrel, Inc.
The enable pin (pin 3) has two threshold levels, allowing
the MIC2196 to shut down in a micro-current mode, or
turn-off output switching in standby mode. Below 0.9V,
the device is forced into a micro power shutdown. If the
enable pin is between 0.9V and 1.5V the output gate
drive is disabled but the internal circuitry is powered on
and the soft start pin voltage is forced low. There is
typically 135mV of hysteresis below the 1.5V threshold
to insure the part does not oscillate on and off due to
ripple voltage on the input. Raising the enable voltage
above the UVLO threshold of 1.5V enables the output
drivers and allows the soft start capacitor to charge. The
enable pin may be pulled up to VINA.
Oscillator and Sync
The internal oscillator is self-contained and requires no
external components. The maximum duty cycle of the
MIC2196 is 85%.
Minimum duty cycle becomes important in a boost
converter as the input voltage approaches the output
voltage. At lower duty cycles, the input voltage can be
closer to the output voltage without the output rising out
of regulation. Minimum duty cycle is typically 7%.
A frequency foldback mode is enabled if the voltage on
the feedback pin (pin 2) is less than 0.3V. In frequency
foldback the oscillator frequency is reduced by
approximately a factor of 4.
Voltage Setting Components
The MIC2196 requires two resistors to set the output
voltage as shown in Figure 5.
MIC2196
Voltage
Amplifier
V_REF
1.245V
R1
Pin
6 _R2
Figure 5. Voltage Setting Components
The output voltage is determined by the equation below:
V_O
=
V_REF
×1+
R1
R2
Where: V_REF for the MIC2196 is nominally 1.245V.
Lower values of resistance are preferred to prevent
noise from appearing on the V_FB pin. A typically
recommended value for R1 is 10K.
MIC2196
Decoupling Capacitor Selection
A 1μF decoupling capacitor is used to stabilize the
internal regulator and minimize noise on the V_DD pin.
Placement of this capacitor is critical to the proper
operation of the MIC2196. It must be next to the V_DD and
signal ground pins and routed with wide etch. The
capacitor should be a good quality ceramic. Incorrect
placement of the V_DD decoupling capacitor will cause
jitter and/or oscillations in the switching waveform as
well as variations in the overcurrent limit.
A minimum 1μF ceramic capacitor is required to
decouple the V_IN. The capacitor should be placed near
the IC and connected directly between pins 8 (V_CC) and
6 (GND). For V_IN greater than 8V, use a 4.7μF or a 10μF
ceramic capacitor to decouple the V_DD 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 VIN 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.
• Use a ferrite material for the inductor core, which
has less core loss than an MPP or iron power
core.
The significant contributors to power loss at higher
output loads are (in approximate order of magnitude):
• Resistive on-time losses in the MOSFET
• Switching transition losses in the MOSFET
• Inductor resistive losses
• Current sense resistor losses
• Output capacitor resistive losses (due to the
capacitor’s ESR)
To minimize power loss under heavy loads:
• Use logic level, low on resistance MOSFETs.
Multiplying the gate charge by the on-resistance
gives a figure of merit, providing a good balance
between switching and resistive power
dissipation.
• Slow transition times and oscillations on the
voltage and current waveforms dissipate more
power during the turn-on and turn-off of the low
side MOSFET. A clean layout will minimize
parasitic inductance and capacitance in the gate
drive and high current paths. This will allow the
September 2008
10
M9999-092908