MIC3172YN Micrel Inc, MIC3172YN Datasheet - Page 14

• Manufacturer Part Number: MIC3172YN
• Description: IC REG SW 100KHZ 1.25A 8-DIP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Cukr

Specifications of MIC3172YN

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 1.25A
• Frequency - Switching: 100kHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 302mW
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Range: 3V To 40V
• Termination Type: Through Hole
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-2255; MIC3172YN

A single point ground is strongly recommended for
proper operation.
The signal ground, compensation network ground, and
feedback network connections are sensitive to minor
voltage variations. The input and output capacitor
grounds and power ground conductors will exhibit
voltage drop when carrying large currents. Keep the
sensitive circuit ground traces separate from the power
ground traces. Small voltage variations applied to the
sensitive circuits can prevent the MIC2172/3172 or any
switching regulator from functioning properly.
Applications and Design Hints
Access to both the collector and emitter(s) of the NPN
power switch makes the MIC2172/3172 extremely
versatile and suitable for use in most PWM power supply
topologies.
Boost Conversion
Refer to figure 11 for a typical boost conversion
application where a +5V logic supply is available but
+12V at 0.14A is required.
The first step in designing a boost converter is
determining whether inductor L1 will cause the converter
to operate in either continuous or discontinuous mode.
Discontinuous mode is preferred because the feedback
control of the converter is simpler.
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in discontinuous
mode.
Micrel
April 2006
Figure 11. 5V to 12V Boost Converter
Figure 10. Single Point Ground
14
L1 is operating in continuous mode if it does not
discharge completely before the MIC2172/3172 power
switch is turned on again.
Discontinuous Mode Design
Given the maximum output current, solve equation (1) to
determine
discontinuous mode without initiating the internal device
current limit.
Where:
For the example in figure 11.
Then:
This value is greater than the 0.14A output current
requirement so we can proceed to find the inductance
value of L1.
Where:
For our practical example:
I
I
I
(Refer to Electrical Characteristics.)
I
V
δ = duty cycle
V
V
I
I
V
δ = 0.623
V
V
P
f
I
I
I
L1 ≤
δ
CL
CL
CL
OUT
OUT
CL
SW
OUT
OUT
OUT
IN
OUT
F
IN
OUT
F
OUT
=
= internal switch current limit
= 1.25A when δ < 50%
= 0.833 (2 – δ ) when δ ≥ 50%
= D1 forward voltage drop
= 1.147A
= 0.6V
= minimum input voltage
= 4.75V (minimum)
= 1⋅10
= maximum output current
= 0.14A
V
= required output voltage
= 12.0V
= 12 ⋅ 0.14 = 1.68W
2
≤
≤
≤
OUT
P
V
(
0.141A
⎛
⎜
⎝
⎛
⎜
⎝
V
OUT
OUT
whether
I
1.147
IN
CL
2
5
V
+
kHz (100kHz)
2
δ
OUT
V
⎞
⎟
⎠
f
)
+
V
SW
F
2
IN
V
⎞
⎟
⎠
±
×
F
δ
V
12
4.75
IN
the
×
0.623
device
can
MIC2172/3172
[phone]
M9999-041806
operate
(1a)
(1)
(2)
in