MIC2199 MICREL [Micrel Semiconductor], MIC2199 Datasheet - Page 13

• Manufacturer Part Number: MIC2199
• Manufacturer: MICREL [Micrel Semiconductor]

The input capacitor must be rated for the input current ripple.
The RMS value of input capacitor current is determined at the
maximum output current. Assuming the peak-to-peak induc-
tor ripple current is low:
The power dissipated in the input capacitor is:
Voltage Setting Components
The MIC2199 requires two resistors to set the output voltage
as shown in Figure 6.
The output voltage is determined by the equation:
Where: V
A typical value of R1 can be between 3k and 10k. If R1 is too
large it may allow noise to be introduced into the voltage
feedback loop. 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 using:
Voltage Divider Power Dissipation
The reference voltage and R2 set the current through the
voltage divider.
The power dissipated by the divider resistors is:
Efficiency Calculation and 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:
November 2004
MIC2199
I
P
R2
I
P
DIVIDER
C (rms)
DISS(C )
DIVIDER
IN
Figure 6. Voltage-Divider Configuration
V
=
REF
O
V
V
O
=
REF
IN
MIC2199
≈
for the MIC2199 is typically 0.8V.
=
V
−
I
=
OUT(max)
REF
=
V
V
(R1 R2) I
×
R2
I
REF
REF
C (rms)
R1
IN
×
+
Error
Amp
⎛
⎜
⎝
1
+
×
2
R2
R1
×
0.8V
×
V
DIVIDER
REF
D (1 D)
R
⎞
⎟
⎠
ESR(C )
×
−
2
IN
FB
3
R1
R2
13
To maximize efficiency at light loads:
Under heavy output loads the significant contributors to
power loss are (in approximate order of magnitude):
To minimize power loss under heavy loads:
• Supply current to the MIC2199
• MOSFET gate-charge power (included in the IC
• Core losses in the output inductor
• Use a low gate-charge MOSFET or use the small-
• Use a ferrite material for the inductor core, which
• Resistive on-time losses in the MOSFETs
• Switching transition losses in the MOSFETs
• Inductor resistive losses
• Current-sense resistor losses
• Input capacitor resistive losses (due to the capaci-
• Use logic-level, low on-resistance MOSFETs. Mul-
• Slow transition times and oscillations on the volt-
• For the same size inductor, a lower value will have
• Lowering the current-sense resistor value will de-
• Use low-ESR input capacitors to minimize the
supply current)
est MOSFET, which is still adequate for maximum
output current.
has less core loss than an MPP or iron power core.
tors ESR)
tiplying the gate charge by the on-resistance gives
a figure of merit, providing a good balance be-
tween low and high load efficiency.
age and current waveforms dissipate more power
during turn-on and turnoff of the MOSFETs. A
clean layout will minimize parasitic inductance and
capacitance in the gate drive and high current
paths. This will allow the fastest transition times
and waveforms without oscillations. Low gate-
charge MOSFETs will transition faster than those
with higher gate-charge requirements.
fewer turns and therefore, lower winding resis-
tance. However, using too small of a value will
require more output capacitors to filter the output
ripple, which will force a smaller bandwidth, slower
transient response and possible instability under
certain conditions.
crease the power dissipated in the resistor. How-
ever, it will also increase the overcurrent limit and
will require larger MOSFETs and inductor compo-
nents.
power dissipated in the capacitors ESR.
MIC2199
Micrel