MIC2172_05 Datasheet, PDF (14/17 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

MIC2172/3172
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
(12) a ≤
Then:
L_PRI
L_SEC
a≤
1.8 × 10^-5
2.54 × 10^-5
a ≤ 0.84 Use 0.8 (same as 1:1.25).
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know the
primary peak current which must be withstood without satu-
rating the transformer core.
(13)
So:
I_PEAK(pri) =
V_IN(min) T_ON
L_PRI
I_PEAK(pri) =
4.0 × 5.5 × 10^-6
18µH
I_PEAK(pri) = 1.22A
Now find the minimum reverse voltage requirement for the
output rectifier. This rectifier must have an average current
rating greater than the maximum output current of 0.25A.
( ) ₍₁₄₎
V_BR ≥
V_IN(max) + V_OUT a
F_BR a
Where:
V_BR = output rectifier maximum peak
reverse voltage rating
Micrel, Inc.
a = transformer turns ratio (0.8)
F_BR = reverse voltage safety derating factor (0.8)
Then:
V_BR ≥
6.0 + (5.0 × 0.8)
0.8 × 0.8
V_BR ≥ 15.625V
A 1N5817 will safely handle voltage and current require-
ments in this example.
Forward Converters
Micrel’s MIC2172/3172 can be used in several circuit con-
figurations to generate an output voltage which is less than
the input voltage (buck or step-down topology). Figure 13
shows the MIC3172 in a voltage step-down application.
Because of the internal architecture of these devices, more
external components are required to implement a step-down
regulator than with other devices offered by Micrel (refer to
the LM257x or LM457x family of buck switchers). However,
for step-down conversion requiring a transformer (forward),
the MIC2172/3172 is a good choice.
A 12V to 5V step-down converter using transformer isolation
(forward) is shown in figure 14. Unlike the isolated flyback
converter which stores energy in the primary inductance
during the controller’s on-time and releases it to the load
during the off-time, the forward converter transfers energy to
the output during the on-time, using the off-time to reset the
transformer core. In the application shown, the transformer
core is reset by the tertiary winding discharging T1’s peak
magnetizing current through D2.
For most forward converters the duty cycle is limited to 50%,
allowing the transformer flux to reset with only two times the
input voltage appearing across the power switch. Although
during normal operation this circuit’s duty cycle is well below
M9999-042205
V_IN
D1
1N4148
C1*
100µF
V_IN
EN
V_SW
C2
2.2µF
MIC3172
COMP _GND FB
R3
P1 P2 S
470
C3
1µF
R3^†
3.7k
R2^†
1.2k
D2
D3
1N4148
C4
R4
1µF
10Ω
L1
100µH
C5
330µF
5V, 0.1A to 1A
(I_LOAD > 100mA)
* Locate near MIC2172/3172 when supply leads > 2"
† _{R3/R2 sets output voltage}
Figure 13. Step-Down or Buck Converter
14
April 2005