MIC2101_13 Datasheet, PDF (25/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
For the low-side MOSFET:
I_{G[LOW -SIDE]} (AVG) C_ISS u V_GS u f_SW
Eq. 6
Since the current from the gate drive comes from the
V_DD, the power dissipated in the MIC2101/02 due to gate
drive is:
P_GATEDRIVE V_DD u (I_G[HIGH-SIDE] (AVG) _{Eq. 7}
 I_{G[LOW -SIDE]} (AVG))
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R_DS(ON) ×
Q_G. Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2101/02. Also, the R_DS(ON) of the low-
side MOSFET will determine the current-limit value.
Please refer to “Current Limit” subsection is Functional
Description for more details.
Parameters that are important to MOSFET switch
selection are:
x Voltage rating
x On-resistance
x Total gate charge
The voltage ratings for the high-side and low-side
MOSFETs are essentially equal to the power stage input
voltage V_HSD. A safety factor of 20% should be added to
the V_DS(MAX) of the MOSFETs to account for voltage
spikes due to circuit parasitic elements.
The power dissipated in the MOSFETs is the sum of the
conduction losses during the on-time (P_CONDUCTION) and
the switching losses during the period of time when the
MOSFETs turn on and off (P_AC).
P_SW P_CONDUCTION  P_AC
Eq.8
P_CONDUCTION I_SW(RMS)2 u R_DS(ON)
Eq. 9
P_AC P_{AC(off )}  P_AC(on)
Eq. 10
where:
R_DS(ON) = On-resistance of the MOSFET switch
D = Duty Cycle = V_OUT / V_HSD
MIC2101/02
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
t_T
C_ISS u V_IN  C_OSS u V_HSD
I_G
Eq.11
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = Gate-drive current
The total high-side MOSFET switching loss is:
P_AC (V_HSD  V_D ) u I_PK u t _T u f_SW
Eq. 12
where:
t_T = Switching transition time
V_D = Body diode drop (0.5V)
f_SW = Switching Frequency
The high-side MOSFET switching losses increase with
the switching frequency and the input voltage V_HSD. The
low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine the
peak-to-peak inductor ripple current. Generally, higher
inductance values are used with higher input voltages.
Larger peak-to-peak ripple currents will increase the
power dissipation in the inductor and MOSFETs. Larger
output ripple currents will also require more output
capacitance to smooth out the larger ripple current.
Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more
expensive inductor.
A good compromise between size, loss and cost is to set
the inductor ripple current to be equal to 20% of the
maximum output current.
November 13, 2013
25
Revision 2.0