MIC33050_1205 Datasheet, PDF (10/16 Pages) Micrel Semiconductor – 4MHz Internal Inductor PWM Buck Regulator with HyperLight Load

Micrel, Inc.
MIC33050
Applications Information
Input Capacitor
A minimum of 2.2µF ceramic capacitor should be placed
close to the VIN pin and PGND pin for bypassing. X5R or
X7R dielectrics are recommended for the input capacitor.
Y5V dielectrics, aside from losing most of their
capacitance over temperature, they also become resistive
at high frequencies. This reduces their ability to filter out
high frequency noise.
Output Capacitor
The MIC33050 was designed for use with a 2.2µF or
greater ceramic output capacitor. A low equivalent series
resistance (ESR) ceramic output capacitor either X7R or
X5R is recommended. Y5V and Z5U dielectric capacitors,
aside from the undesirable effect of their wide variation in
capacitance over temperature, become resistive at high
frequencies.
Compensation
The MIC33050 is designed to be stable with an internal
inductor with a minimum of 2.2µF ceramic (X5R) output
capacitor.
Efficiency Considerations
Efficiency is defined as the amount of useful output power,
divided by the amount of power supplied.
Efficiency
(%)


V_OUT
V_IN


I _OUT
I _IN
 ^100
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design considerations
and it reduces consumption of current for battery powered
applications. Reduced current draw from a battery
increases the devices operating time and is critical in hand
held devices.
There are two types of losses in switching converters; DC
losses and switching losses. DC losses are simply the
power dissipation of I²R. Power is dissipated in the high
side switch during the on cycle. Power loss is equal to the
high side MOSFET R_DSON multiplied by the square of the
Switch Current. During the off cycle, the low side N-
channel MOSFET conducts, also dissipating power.
Device operating current also reduces efficiency. The
product of the quiescent (operating) current and the supply
voltage is another DC loss. The current required driving
the gates on and off at a constant 4MHz frequency and the
switching transitions make up the switching losses.
Efficiency V = 1.8V
OUT
100
90
V_IN = 3.0V
80
70 _{VIN = 3.6V}
60
V_IN = 4.2V
501
10
100
1000
OUTPUT CURRENT (mA)
The Figure above shows an efficiency curve. From 1µA to
100mA, efficiency losses are dominated by quiescent
current losses, gate drive and transition losses. By using
the HyperLight Load^® mode, the MIC33050 is able to
maintain high efficiency at low output currents.
Over 100mA, efficiency loss is dominated by MOSFET
RDSON and inductor losses. Higher input supply voltages
will increase the Gate-to-Source threshold on the internal
MOSFETs, thereby reducing the internal RDSON. This
improves efficiency by reducing DC losses in the device.
All but the inductor losses are inherent to the device. In
which case, inductor selection becomes increasingly
critical in efficiency calculations. As the inductors are
reduced in size, the DC resistance (DCR) can become
quite significant. The DCR losses can be calculated as
follows:
L_PD = I_OUT2 × DCR
From that, the loss in efficiency due to inductor resistance
can be calculated as follows;
Efficiency
Loss
(%)


^{1 }


V_OUT  I _OUT
V_OUT  I _OUT  L _PD

100
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and size
in this case.
May 2012
10
M9999-050312-C