MIC2174-1YMM Micrel Inc, MIC2174-1YMM Datasheet - Page 19

• Manufacturer Part Number: MIC2174-1YMM
• Description: IC BUCK SYNC ADJ 25A 10MSOP
• Manufacturer: Micrel Inc
• Series: Hyper Speed Control™r
• Type: Step-Down (Buck)r

Specifications of MIC2174-1YMM

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5.5 V
• Frequency - Switching: 300kHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Power - Output: 421mW
• Primary Input Voltage: 40V
• No. Of Outputs: 2
• Output Current: 25A
• Voltage Regulator Case Style: MSOP
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC (15-Dec-2010)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-3546-5

Micrel, Inc.
2. Inadequate ripple at the FB voltage due to the small
3. Virtually no ripple at the FB pin voltage is due to the
September 2010
ESR of the output capacitors.
The output voltage ripple is fed into the FB pin
through a feedforward capacitor C
as shown in Figure 7b. The typical C
between 1nF and 100nF. With the feedforward
capacitor, the FB voltage ripple is very close to the
output voltage ripple:
very low ESR of the output capacitors.
Figure 7b. Inadequate Ripple at FB
ΔV
Figure 7c. Invisible Ripple at FB
Figure 7a. Enough Ripple at FB
FB(pp)
≈
ESR
×
ΔI
L
(pp)
ff
in this situation,
ff
value is
(36)
19
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node LX via a resistor R
capacitor C
is:
where
V
D = Duty Cycle
f
τ = (R1//R2//R
In the equations (37) and (38), it is assumed that the
time constant associated with C
than the switching period:
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C
time constant consumption. Also, a 100nF injection
capacitor C
for a wide range of the frequencies.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C
100nF if R1 and R2 are in kΩ range.
Step 2. Select R
voltage ripple. According to Equation 37:
Then the value of Rinj is obtained as:
SW
HSD
= switching frequency
ΔV
= Power stage input voltage at HSD pin
FB(pp)
ff
inj
inj
K
of 1nF to 100nF can easily satisfy the large
K
R
, as shown in Figure 7c. The injected ripple
=
is used in order to be considered as short
div
inj
div
inj
V
) × C
HSD
f
=
=
=
SW
inj
ff
(R1//R2)
R
ΔV
1
according to the expected feedback
×
inj
ff
×
to feed all output ripples into the
V
R1//R2
K
τ
HSD
FB(pp)
+
div
=
R1//R2
T
τ
×
×
×
D
<<
(
D
×
K
1
(1
f
×
SW
1
div
ff
-
(1
D)
must be much greater
×
MIC2174/MIC2174C
−
−
×
τ
D)
1)
f
SW
M9999-091310-C
1
×
τ
ff
is 1nF to
inj
and a
(37)
(38)
(39)
(40)