MIC23303 Datasheet, PDF (13/21 Pages) Micrel Semiconductor – 4MHz PWM 3A Buck Regulator with HyperLight Load™ and Power Good

Micrel, Inc.
MIC23303
Figure 2. HyperLight Load (HLL) and Continuous
Conduction Mode (CCM) Switching Diagram
Figure 2 shows the signals for high-side switch drive
(HSD) for Ton control, the inductor current and the low-
side switch drive (LSD) for Toff control.
In HLL mode, the inductor is charged with a fixed Ton
pulse on the high-side switch (HSD). After this, the LSD is
switched on and current falls at a rate V_OUT/L. The
controller remains in HLL mode while the inductor falling
current is detected to cross approximately 300mA. When
the LSD (or Toff) time reaches its minimum and the
inductor falling current is no longer able to reach this
300mA threshold, the part is in CCM mode and switching
at a virtually constant frequency.
Compensation
The MIC23303 is designed to be stable with a 0.33µH to
1.0µH inductor with a minimum 10µF ceramic (X5R) output
capacitor. The total feedback resistance should be kept
around 500kΩ to reduce the I²R losses through the
feedback resistor network, improving efficiency. A feed-
forward capacitor (CFF) of 33pF is recommended across
the top feedback resistor to reduce the effects of parasitic
capacitance and improve transient performance.
Duty Cycle
The typical maximum duty cycle of the MIC23303 is 85%.
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
handheld 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 switch current
squared. 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
represents another DC loss.
The current required to drive the gates on and off at a
constant 4MHz frequency and the switching transitions
make up the switching losses.
Efficiency vs. Load
1.8 VOUT
100
90
80
70
VIN = 3.6V
60
VIN = 5V
50
40
30
20
10
0
0.0001
L = 0.33µH
COUT = 44µF
0.001 0.01
0.1
1
10
LOAD CURRENT(A)
Figure 3. Efficiency under Load
Figure 3 shows an efficiency curve. From no load to
100mA, efficiency losses are dominated by quiescent
current losses, gate drive, and transition losses. By using
HyperLight Load mode, the MIC23303 is able to maintain
high efficiency at low output currents.
Over 300mA, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply voltages
will increase the Gate-to-Source voltage on the internal
MOSFETs, thereby reducing the internal R_DSON. This
improves efficiency by reducing DC losses in the device.
All but the inductor losses are inherent to the device.
When dealing with inductor losses, inductor selection
becomes increasingly critical in efficiency calculations.
September 6, 2013
13
090613-2.0