MIC23603 Datasheet, PDF (13/19 Pages) Micrel Semiconductor – 4MHz PWM 6A Buck Regulator

Micrel Inc.
MIC23603
Device operating current also reduces efficiency. The
product of the quiescent (operating) current and the
supply voltage represents another DC loss. The current
needed to drive the gates on and off at a constant 4MHz
frequency and the switching transitions make up the
switching losses.
Efficiency vs.
Output Current V_OUT = 2.5V
100
90
80
70
60
50
40
30
20
10
0
0.0001
0.001
0.01
V_IN = 5V
L = 0.33µH
C_OUT = 2x47µF
0.1
1
10
OUTPUT CURRENT (A)
Figure 2. Efficiency Under Load
Figure 2 shows an efficiency curve, from no load to
300mA. Efficiency losses are dominated by quiescent
current losses, gate drive, and transition losses. By
using the HyperLight Load mode, the MIC23603 can
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 threshold on the
internal MOSFETs, which reduces the internal R_DSON.
This improves efficiency by reducing DC losses in the
device. All but the inductor losses are inherent to the
device. In this case, inductor selection becomes
increasingly critical in efficiency calculations. As the
inductors get smaller, the DC resistance (DCR) can
become quite significant. The DCR losses can be
calculated as follows:
P_DCR = I_OUT2 × DCR
Eq. 4
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 + P_DCR
 ^{× 100 Eq. 5}
Efficiency loss caused by DCR is minimal at light loads
and gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size.
HyperLight Load^® Mode
MIC23603 uses a minimum on and off time proprietary
control loop (patented by Micrel). When the output
voltage falls below the regulation threshold, the error
comparator begins a switching cycle that turns the
PMOS on and keeps it on for the duration of the
minimum-on-time. This increases the output voltage. If
the output voltage is over the regulation threshold, then
the error comparator turns the PMOS off for a minimum-
off-time until the output drops below the threshold. The
NMOS acts as an ideal rectifier that conducts when the
PMOS is off. Using an NMOS switch instead of a diode
allows for lower voltage drop across the switching device
when it is on. The asynchronous switching combination
between the PMOS and the NMOS allows the control
loop to work in discontinuous mode for light load
operations. In discontinuous mode, the MIC23603 works
in pulse frequency modulation (PFM) to regulate the
output. As the output current increases, the off-time
decreases, which provides more energy to the output.
This switching scheme improves the efficiency of
MIC23603 during light load currents by switching only
when needed. As the load current increases, the
MIC23603 goes into continuous conduction mode (CCM)
and switches at a frequency centered at 4MHz. The load
when the MIC23603 goes into continuous conduction
mode may be approximated by the following formula:
I_LOAD
>
 ^(VIN
− V_OUT
2L × f
)
×
D

Eq. 6
As shown in the previous equation, the load at which
MIC23603 transitions from HyperLight Load mode to
PWM mode is a function of the input voltage (V_IN), output
voltage (V_OUT), duty cycle (D), inductance (L), and
frequency (f). As shown in Figure 3, as the Output
Current increases, the switching frequency also
increases, until the MIC23603 goes from HyperLight
Load mode to PWM mode at approximately 300mA. The
MIC23603 switches a relatively constant frequency
around 4MHz after the output current is over 300mA.
10000
Switching Frequency vs.
Load Current
1000
V_IN = 2.9V
100
10
V_IN = 3.6V
1
V_IN=5V
V_OUT = 1.8V
L = 0.33µH
C_OUT = 2x47µF
0.1
0.0001 0.001 0.01
0.1
1
10
LOAD CURRENT (A)
Figure 3. SW Frequency vs. Output Current
November 5, 2013
13
Revision 1.1