MIC2168_05 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

MIC2168
V_DD regulator operates in dropout mode, and it is necessary
that the power MOSFETs used are low threshold and are in
full conduction mode for V_GS of 2.5V. For applications when
V_IN > 5V; logic-level MOSFETs, whose operation is specified
at V_GS = 4.5V must be used.
It is important to note the on-resistance of a MOSFET increases
with increasing temperature. A 75°C rise in junction tempera-
ture will increase the channel resistance of the MOSFET by
50% to 75% of the resistance specified at 25°C. This change
in resistance must be accounted for when calculating MOSFET
power dissipation and in calculating the value of current-sense
(CS) resistor. 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 MIC2168
gate drive circuit. At 1MHz switching frequency and above, the
gate charge can be a significant source of power dissipation
in the MIC2168. 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 usu-
ally 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:
I_{G[low -side](avg)} = C _ISS × V_GS × f_S
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2168 due to gate
drive is:
( ) ^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
MIC2168.
Parameters that are important to MOSFET switch selection
are:
• Voltage rating
• On-resistance
• Total gate charge
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)
Micrel, Inc.
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
I_G
× V_IN
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = gate-drive current (1A for the MIC2168)
The total high-side MOSFET switching loss is:
P_AC = (V_IN +V_D ) × I_PK × t_T × f_S
where:
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.
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 induc-
tor 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 cur-
rent. 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. The inductance value is calculated
by the equation below.
L = ^{VOUT × (VIN (max ) − VOUT )}
V_IN (max ) × f_S × 0.2 × I_OUT (max )
where:
f_S = switching frequency, 1MHz
0.2 = ratio of AC ripple current to DC output current
V_IN(max) = maximum input voltage
The peak-to-peak inductor current (AC ripple current) is:
M9999-040805
8
April 2005