SC174MLTRT Semtech, SC174MLTRT Datasheet - Page 18

• Manufacturer Part Number: SC174MLTRT
• Description: IC BUCK SYNC ADJ 4A MLPD-10
• Manufacturer: Semtech
• Series: EcoSpeed™r
• Type: Step-Down (Buck)r

Specifications of SC174MLTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.23 V
• Current - Output: 4A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MLPD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC174MLTR

Applications Information (continued)
and discharging during the switching cycle. For most ap-
plications, the total output ripple voltage is dominated
by the output capacitors, typically SP or POSCAP devices.
For stability the ESR zero of the output capacitor should
be lower than approximately one-third the switching fre-
quency. The formula for minimum ESR is shown by the
following equation.
Using Ceramic Output Capacitors
When applications use ceramic output capacitors, the
ESR is normally too small to meet the previously stated
ESR criteria. In these applications it is necessary to add
a small signal injection network as shown in Figure 9. In
this network R
generate an in-phase ripple voltage comparable to the
ripple seen on higher ESR capacitors. C
pacitor used to AC couple the generated ripple onto the
FB pin. Capacitor C
This capacitor introduces a lead/lag into the control with
the maximum phase placed at 1/2 f
The values of R
ditions of the specific application such as V
I
800kHz, calculations plus experimental test results show
that the following combination of R
C
ages and loads.
© 2010 Semtech Corporation
OUT
C
=68pF and C
. For switching frequencies ranging from 600kHz to
Q2
Q1
Figure 9 — Signal Injection Circuit
V
L
V
FF
L
and C
LX
IN
, C
=39pF can be used for many output volt-
ESR
L
, C
FF
is required for min C
C
L
C
R
MIN
filter the LX switching waveform to
and C
L
L
=
L
2
C
FF
×
C
are dependent on the con-
π
×
C
C
FF
3
OUT
SW
for added stability.
L
×
C
=2.5kW, C
is a coupling ca-
f
SW
OUT
R2
R1
IN
applications.
, V
OUT
C
, f
L
OUT
=10nF,
SW
and
Output Voltage Dropout
The output voltage adjustable range for continuous-
conduction operation is limited by the fixed 320ns (typi-
cal) minimum off-time. When working with low input
voltages, the duty-factor limit must be calculated using
worst-case values for on and off times. The duty-factor
limitation is shown by the next equation.
The inductor resistance and MOSFET on-state voltage
drops must be included when performing worst-case
dropout duty-factor calculations.
System DC Accuracy — V
Three factors affect V
error comparator, the ripple voltage variation with line
and load, and the external resistor tolerance. The error
comparator offset is trimmed so that under static condi-
tions it trips when the feedback pin is 750mV,
The on-time pulse from the SC174 in the design example
is calculated to give a pseudo-fixed frequency of 800kHz.
Some frequency variation with line and load is expected.
This variation changes the output ripple voltage. Be-
cause adaptive on-time converters regulate to the valley
of the output ripple, ½ of the output ripple appears as a
DC regulation error. For example, if the output ripple is
50mV with V
will be 25mV above the comparator trip point. If the rip-
ple increases to 30mV with V
DC output will be 15mV above the comparator trip. The
best way to minimize this effect is to minimize the output
ripple.
To compensate for valley regulation, it may be desirable
to use passive droop. Take the feedback directly from the
output side of the inductor and place a small amount of
trace resistance between the inductor and output ca-
pacitor. This trace resistance should be optimized so that
at full load the output droops to near the lower regula-
tion limit. Passive droop minimizes the required output
capacitance because the voltage excursions due to load
steps are reduced as seen at the load.
IN
= 5 volts, then the measured DC output
DUTY
OUT
accuracy: the trip point of the FB
OUT
T
ON
IN
Controller
(
MIN
= 5.5V, then the measured
T
)
ON
(
MIN
T
OFF
)
(
MAX
)
SC174
+
1%.
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