MIC2193 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – 400KHZ SO 8 SYNCHRONOUS BUCK CONTROL IC

MIC2193
MOSFET Selection
The P-channel MOSFET must have a V_GS threshold voltage
equal to or lower than the input voltage when used in a buck
converter topology. There is a limit to the maximum gate
charge the MIC2193 will drive. MOSFETs with higher gate
charge will have slower turn-on and turn-off times. Slower
transition times will cause higher power dissipation in the
MOSFETs due to higher switching transition losses. The
MOSFETs must be able to completely turn on and off within
the driver non-overlap time If both MOSFETs are conducting
at the same time, shoot-through will occur, which greatly
increases power dissipation in the MOSFETs and reduces
converter efficiency.
The MOSFET gate charge is also limited by power dissipation
in the MIC2193. The power dissipated by the gate drive
circuitry is calculated below:
P_{GATE_DRIVE} = Q_GATE × V_IN × f_S
where:
Q_GATE is the total gate charge of both the N and P-
channel MOSFETs.
f_S is the switching frequency
V_IN is the gate drive voltage
The graph in Figure 3 shows the total gate charge that can be
driven by the MIC2193 over the input voltage range, for
different values of switching frequency.
Max. Gate Charge
100
90
80
70
60
50
40
30
20
10
00
2 4 6 8 10 12 14
INPUT VOLTAGE (V)
Figure 3. MIC2193 Frequency vs Max. Gate Charge
Oscillator
The internal oscillator is free running and requires no external
components. The maximum duty cycle is 100%. This is
another advantage of using a P-channel MOSFET for the
high-side drive: it can continuously turned on.
A frequency foldback mode is enabled if the voltage on the
feedback pin (pin 3) is less than 0.3V. In frequency foldback,
the oscillator frequency is reduced by approximately a factor
of 4. Frequency foldback is used to limit the energy delivered
to the output during a short circuit fault condition.
Voltage Setting Components
The MIC2193 requires two resistors to set the output voltage
as shown in Figure 4.
MIC2193
V_OUT
Voltage
Amplifier
R1
Pin 3
R2
V_REF
1.245V
Micrel
Figure 4
The output voltage is determined by the equation below.
V_OUT =
V_REF
×
1+
R1
R2
Where: V_REF for the MIC2193 is typically 1.245V.
Lower values of R1 are preferred to prevent noise from
appearing on the FB pin. A typically recommended value is
10kΩ. If R1 is too small in value it will decrease the efficiency
of the power supply, especially at low output loads.
Once R1 is selected, R2 can be calculated with the following
formula.
R2= ^{VREF × R1}
V_OUT – V_REF
Efficiency Considerations
Efficiency is the ratio of output power to input power. The
difference is dissipated as heat in the buck converter. Under
light output load, the significant contributors are:
• The V_IN supply current
To maximize efficiency at light loads:
• Use a low gate charge MOSFET or use the smallest
MOSFET, which is still adequate for maximum output
current.
• Use a ferrite material for the inductor core, which has
less core loss than an MPP or iron power core.
Under heavy output loads the significant contributors to
power loss are (in approximate order of magnitude):
• Resistive on time losses in the MOSFETs
• Switching transition losses in the high side MOSFET
• Inductor resistive losses
• Current sense resistor losses
• Input capacitor resistive losses (due to the capacitors
ESR)
To minimize power loss under heavy loads:
• Use low on resistance MOSFETs. Use low threshold
logic level MOSFETs when the input voltage is below
5V. Multiplying the gate charge by the on resistance
gives a figure of merit, providing a good balance
between low load and high load efficiency.
• Slow transition times and oscillations on the voltage
and current waveforms dissipate more power during
the turn on and turn off of the MOSFETs. A clean
layout will minimize parasitic inductance and capaci-
tance in the gate drive and high current paths. This
will allow the fastest transition times and waveforms
without oscillations. Low gate charge MOSFETs will
M9999-042704
8
April 2004