LM20145MHE/NOPB National Semiconductor, LM20145MHE/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM20145MHE/NOPB
• Description: IC REG SYNC BUCK 5A ADJ 16-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM20145MHE/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5 V
• Current - Output: 5A
• Frequency - Switching: 250kHz ~ 750kHz
• Voltage - Input: 2.95 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Power - Output: 2.6W
• For Use With: LM20145EVAL - BOARD EVAL 5A POWERWISE LM20145
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM20145MHE

Where, C
L (H) is the value of the inductor, V
voltage drop ignoring loop bandwidth considerations, ΔI
STEP
capacitor ESR, V
the set regulator output voltage. Both the tolerance and volt-
age coefficient of the capacitor needs to be examined when
designing for a specific output ripple or transient drop target.
INPUT CAPACITOR SELECTION (C
Good quality input capacitors are necessary to limit the ripple
voltage at the VIN pin while supplying most of the switch cur-
rent during the on-time. In general it is recommended to use
a ceramic capacitor for the input as they provide both a low
impedance and small footprint. One important note is to use
a good dielectric for the ceramic capacitor such as X5R or
X7R. These provide better over temperature performance
and also minimize the DC voltage derating that occurs on Y5V
capacitors. For most applications, a 22 µF, X5R, 6.3V input
capacitor is sufficient; however, additional capacitance may
be required if the connection to the input supply is far from the
PVIN pins. The input capacitor should be placed as close as
possible PVIN and PGND pins of the device.
Non-ceramic input capacitors should be selected for RMS
current rating and minimum ripple voltage. A good approxi-
mation for the required ripple current rating is given by the
relationship:
As indicated by the RMS ripple current equation, highest re-
quirement for RMS current rating occurs at 50% duty cycle.
For this case, the RMS ripple current rating of the input ca-
pacitor should be greater than half the output current. For best
performance, low ESR ceramic capacitors should be placed
in parallel with higher capacitance capacitors to provide the
best input filtering for the device.
SETTING THE OUTPUT VOLTAGE (R
The resistors R
voltage for the device. Table 1, shown below, provides sug-
gestions for R
If different output voltages are required, R
lected to be between 4.99 kΩ to 49.9 kΩ and R
calculated using the equation below.
ADJUSTING THE OPERATING FREQUENCY (R
The operating frequency of the LM20145 can be adjusted by
connecting a resistor from the RT pin to ground. The equation
(A) is the load step change, R
TABLE 1. Suggested Values for R
OUT
(F) is the minimum required output capacitance,
FB1
R
FB1
FB1
short
4.99
8.87
12.7
21.5
31.6
IN
and R
(kΩ)
and R
(V) is the input voltage, and V
FB2
R
FB2
for common output voltages.
FB2
open
10.2
10.2
10.2
10.2
10
are selected to set the output
(kΩ)
DROOP
IN
ESR
)
V
FB1
0.8
1.2
1.5
1.8
2.5
3.3
OUT
FB1
, R
(Ω) is the output
FB2
(V) is the output
and R
FB2
should be se-
)
FB1
T
OUT
)
FB2
can be
(V) is
OUT-
13
shown below can be used to calculate the value of R
given operating frequency.
Where, f
frequency adjust resistor in kΩ. Please refer to the curve Os-
cillator Frequency verses R
acteristics section If the R
not operate.
LOOP COMPENSATION (R
The purpose of loop compensation is to meet static and dy-
namic performance requirements while maintaining adequate
stability. Optimal loop compensation depends on the output
capacitor, inductor, load, and the device itself. Table 2 below
gives values for the compensation network that will result in
a stable system when using a 100 µF, 6.3V ceramic X5R out-
put capacitor and 1 µH inductor.
If the desired solution differs from the table above the loop
transfer function should be analyzed to optimize the loop
compensation. The overall loop transfer function is the prod-
uct of the power stage and the feedback network transfer
functions. For stability purposes, the objective is to have a
loop gain slope that is -20db/decade from a very low frequen-
cy to beyond the crossover frequency. Figure 4, shown below,
shows the transfer functions for power stage, feedback/com-
pensation network, and the resulting closed loop system for
the LM20145.
TABLE 2. Recommended Compensation for
C
SW
OUT
is the switching frequency in kHz, and R
5.00
5.00
5.00
5.00
5.00
5.00
3.30
3.30
3.30
3.30
3.30
V
= 100 µF, L = 2.5 µH & f
IN
V
3.30
2.50
1.80
1.50
1.20
0.80
2.50
1.80
1.50
1.20
0.80
OUT
T
T
resistor is omitted the device will
C1
in the typical performance char-
C
, C
C1
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
C1
(nF) R
)
SW
= 250 kHz
C1
9.53
6.98
5.36
4.87
1.91
10.5
7.87
5.62
4.42
2.26
11
(kΩ)
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T
T
is the
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