MIC2182-3.3YSM Micrel Inc, MIC2182-3.3YSM Datasheet - Page 19

• Manufacturer Part Number: MIC2182-3.3YSM
• Description: IC CTRLR SYNC BUCK 3.3V 16-SSOP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck)r

Specifications of MIC2182-3.3YSM

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 20A
• Frequency - Switching: 300kHz
• Voltage - Input: 4.5 ~ 16.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Power - Output: 270mW
• Number Of Pwm Outputs: 1
• On/off Pin: Yes
• Adjustable Output: No
• Topology: Boost/Buck
• Switching Freq: 330KHz
• Operating Supply Voltage (max): 32V
• Output Current: 20mA
• Output Voltage: 3.3V
• Synchronous Pin: Yes
• Rise Time: 60ns
• Fall Time: 60ns
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 16
• Package Type: SSOP
• Duty Cycle: 86%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 576-2157; MIC2182-3.3YSM

The input voltage ripple will primarily depend on the input
capacitors ESR. The peak input current is equal to the peak
inductor current, so:
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 MIC2182-3.3 and MIC2182-5.0 ICs contain internal
voltage dividers that set the output voltage. The MIC2182
adjustable version requires two resistors to set the output
voltage as shown in Figure 13.
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:
April 22, 2004
MIC2182
I
P
R2
I
P
C
divider
DISS(C )
divider
V
Figure 13. Voltage-Divider Configuration
IN
IN
(rms) I
V
REF
O
V
V
O
I
REF
IN
MIC2182 [adj.]
inductor(peak)
for the MIC2182 is typically 1.245V.
V
V
(R1 R2) I
R2
REF
REF
V
OUT(max)
I
REF
C
R1
IN
Error
Amp
(rms)
1
1.245V
R2
R1
divider
2
R
V
REF
ESR(C )
D (1 D)
R
ESR(C )
2
IN
IN
FB
7
R1
R2
19
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:
Decoupling Capacitor Selection
The 4.7 F decoupling capacitor is used to minimize noise on
the VDD pin. The placement of this capacitor is critical to the
proper operation of the IC. It must be placed right next to the
• Supply current to the MIC2182
• MOSFET gate-charge power (included in the IC
• Core losses in the output inductor
• Use a low gate-charge MOSFET or use the
• Allow the MIC2182 to run in skip mode at lower
• 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
• Use logic-level, low on-resistance MOSFETs.
• Slow transition times and oscillations on the
• For the same size inductor, a lower value will
• Lowering the current-sense resistor value will
• Use low-ESR input capacitors to minimize the
supply current)
smallest MOSFET, which is still adequate for
maximum output current.
currents.
has less core loss than an MPP or iron power
core.
capacitors ESR)
Multiplying the gate charge by the on-resistance
gives a Figure of merit, providing a good bal-
ance between low and high load efficiency.
voltage 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 oscilla-
tions. Low gate-charge MOSFETs will transition
faster than those with higher gate-charge
requirements.
have fewer turns and therefore, lower winding
resistance. However, using too small of a value
will require more output capacitors to filter the
output ripple, which will force a smaller band-
width, slower transient response and possible
instability under certain conditions.
decrease the power dissipated in the resistor.
However, it will also increase the overcurrent
limit and will require larger MOSFETs and
inductor components.
power dissipated in the capacitors ESR.
M9999-042204
Micrel