mic33050 Micrel Semiconductor, mic33050 Datasheet - Page 9

• Manufacturer Part Number: mic33050
• Description: Mic33050 4mhz Internal Inductor Pwm Buck Regulator With Hyper Light Load?
• Manufacturer: Micrel Semiconductor

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
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.
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 I
side switch during the on cycle. Power loss is equal to the
high side MOSFET R
Current
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.
Micrel, Inc.
October 2007
dielectrics,
2
. During the off cycle, the low side N-channel
Efficiency
_%
aside
=
2
R. Power is dissipated in the high
⎛
⎜ ⎜
⎝
DSON
V
OUT
V
from
IN
multiplied by the Switch
×
×
I
I
IN
OUT
losing
⎞
⎟ ⎟
⎠
×
100
most
of
their
9
The Figure above shows an efficiency curve. From no load
to 100mA, efficiency losses are dominated by quiescent
current losses, gate drive and transition losses. By using
the Hyper Light Load
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;
From that, the loss in efficiency due to inductor resistance
can be calculated as follows;
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.
Efficiency
L_Pd = I
_Loss
OUT
2
=
× DCR
TM
⎡
⎢
⎢
⎣
1
−
mode, the MIC33050 is able to
⎛
⎜
⎜
⎝
V
OUT
V
OUT
×
I
OUT
×
I
OUT
+
L_Pd
M9999-100407-A
⎞
⎟
⎟
⎠
MIC33050
⎤
⎥
⎥
⎦
×
100