MIC2182 Datasheet, PDF (17/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
charge can be a significant source of power dissipation in the
MIC2182. At low output load this power dissipation is notice-
able as a reduction in efficiency. The average current re-
quired 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
with 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 losses for the low-side MOSFET is
usually negligable. Also, the gate drive current for the low-
side MOSFET is more accurately calculated using C_ISS 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 MIC2182 due to gate
drive is:
( ) ^Pgate drive ^{= 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
MIC2182. Power dissipation in the MIC2182 package limits
the maximum gate drive current. Refer to Figure 10 for the
MIC2182 gate drive limits.
Parameters that are important to MOSFET switch selection
are:
• Voltage rating
• On-resistance
• Total gate charge
The voltage rating of the MOSFETs 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.
Making the assumption the turn-on and turnoff transition
times are equal, the transition time can be approximated by:
Micrel
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 MIC2182)
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 300kHz
The low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
RMS Current and MOSFET Power Dissipation Calculation
Under normal operation, the high-side MOSFET’s RMS
current is greatest when V_IN is low (maximum duty cycle). The
low-side MOSFET’s RMS current is greatest when V_IN is high
(minimum duty cycle). However, the maximum stress the
MOSFETs see occurs during short circuit conditions, where
the output current is equal to I_{overcurrent(max)}. (See the Sense
Resistor section). The calculations below are for normal
operation. To calculate the stress under short circuit condi-
tions, substitute I_{overcurrent(max)} for I_OUT(max). Use the formula
below to calculate D under short circuit conditions.
D_{short circuit} = 0.063 − 1.8 × 10^−3 × V_IN
The RMS value of the high-side switch current is:
I_SW(highside)(rms) =
D
×
_IOUT(max)2
+
I_PP2
12


I_{SW(low side)}(rms) =
(1−
D)
_IOUT(max)2
+
I_PP2
12


where:
D = duty cycle of the converter
D = ^VOUT
η × V_IN
η = efficiency of the converter.
Converter efficiency depends on component parameters,
which have not yet been selected. For design purposes, an
efficiency of 90% can be used for V_IN less than 10V and 85%
can be used for V_IN greater than 10V. The efficiency can be
more accurately calculated once the design is complete. If the
assumed efficiency is grossly inaccurate, a second iteration
through the design procedure can be made.
For the high-side switch, the maximum dc power dissipation
is:
P_switch1(dc) = R_DS(on)1 × I_SW1(rms)²
June 2000
17
MIC2182