MIC2168A Datasheet, PDF (10/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
Total gate charge is the charge required to turn the
MOSFET on and off under specified operating
conditions (V_DS and V_GS). The gate charge is supplied
by the MIC2168A gate-drive circuit. At 1MHz switching
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2168A.
At low output load, this power dissipation is noticeable
as a reduction in efficiency.
The average current required to drive the high-side
MOSFET is:
I_{G[high−side](avg)} = Q_G × f_S
where:
I_{G[high-side](avg)} = average high-side MOSFET gate
current.
Q_G = total gate charge for the high-side MOSFET taken
from manufacturer’s data sheet for V_GS = 5V.
The low-side MOSFET is turned on and off at V_DS = 0
because the freewheeling diode is conducting during
this time. The switching loss for the low-side MOSFET is
usually negligible. Also, the gate-drive current for the
low-side MOSFET is more accurately calculated using
CISS at V_DS = 0 instead of gate charge.
For the low-side MOSFET:
The voltage ratings for the top and bottom MOSFET are
essentially equal to the input voltage. A safety factor of
20% should be added to the V_DS(max) of the MOSFETs
to account for voltage spikes due to circuit parasitics.
The power dissipated in the switching transistor is the
sum of the conduction losses during the on-time
(P_CONDUCTION) and the switching losses that occur during
the period of time when the MOSFETs turn on and off
(P_AC).
P_SW = P_CONDUCTION + P_AC
where:
P_CONDUCTION = I_SW(rms)2 × R_SW
P_AC = P_AC(off) + P_AC(on)
R_SW = on − resistance of the MOSFET switch
D
=
duty
cycle_{⎜⎜⎝⎛}
V_O
V_IN
⎟⎟⎠⎞
Making the assumption the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
t_T
=
C_ISS × V_GS + C_OSS × V_IN
I_G
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = gate-drive current (1A for the MIC2168A)
I_{G[low −side](avg)} = C_ISS × V_GS × f_S
The total high-side MOSFET switching loss is:
Since the current from the gate drive comes from the
input voltage, the power dissipated in the MIC2168A
due to gate drive is:
P_AC = (V_IN + V_D) × I_PK × t_T × f_S
where:
( ) P_GATEDRIVE = V_IN I_{G[high−side](avg)} + 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 MIC2168A.
Parameters that are important to MOSFET switch
selection are:
• Voltage rating
• On-resistance
• Total gate charge
t_T = switching transition time (typically 20ns to 50ns)
V_D = freewheeling diode drop, typically 0.5V
f_S it the switching frequency, nominally 1MHz
The low-side MOSFET switching losses are negligible
and can be ignored for these calculations.
January 2010
10
M9999-011510