MIC2199_10 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 300kHz 4mm × 4mm Synchronous Buck Converter

Micrel, Inc.
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.
f_s = 300kHz
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 negli-
gible. 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 MIC2199 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 MIC2199. Power dissipation in the MIC2199 package
limits the maximum gate drive current.
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:
t_T
=
C_ISS
×
V_GS + C_OSS
I_G
×
V_IN
MIC2199
where:
C_ISS and C_OSS are measured at V_DS = 0.
I_G = gate drive current (1A for the MIC2199)
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 MOSFETs RMS cur-
rent is greatest when V_IN is low (maximum duty cycle). The
low-side MOSFETs 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
conditions, substitute I_{OVERCURRENT(max)} for I_OUT(max). Use
the formula below to calculate D under short circuit condi-
tions.
D_SHORTCIRCUIT = 0.063 −1.8 × 10^−3 × V_IN
The RMS value of the high-side switch current is:
I_{SW(high−side)(rms)} =
D
×
_IOUT(max)2
+
I_PP2
12



I_{SW(low−side)(rms)} =
(1−D)



I_OUT(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 dissipa-
tion is:
P_SWITCH1(dc) = R_DS(on)1× I_{SW1(rms) 2}
January 2010
10
M9999-011310