LM20146MH National Semiconductor, LM20146MH Datasheet - Page 15

• Manufacturer Part Number: LM20146MH
• Description: REG, 6A, SYNC STEP DOWN, 16TSSOP
• Manufacturer: National Semiconductor

Specifications of LM20146MH

• Primary Input Voltage: 5.5V
• No. Of Outputs: 1
• Output Voltage: 5V
• Output Current: 6A
• Voltage Regulator Case Style: TSSOP
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The power stage transfer function is dictated by the modula-
tor, output LC filter, and load; while the feedback transfer
function is set by the feedback resistor ratio, error amp gain,
and external compensation network.
To achieve a -20dB/decade slope, the error amplifier zero,
located at f
pole (f
can be added to cancel the output filter zero at f
cellation of the output filter zero is recommended if larger
value, non-ceramic output capacitors are used.
Compensation of the LM20146 is achieved by adding an RC
network as shown in Figure 6 below.
A good starting value for C
Once the value of C
calculated using the equation below to cancel the output filter
pole (F
A higher crossover frequency can be obtained, usually at the
expense of phase margin, by lowering the value of C
FIGURE 6. Compensation Network for LM20146
P(FIL)
P(FIL)
FIGURE 5. LM20146 Loop Compensation
). An additional error amp pole, located at f
Z(EA)
) as shown in Figure 5.
, should positioned to cancel the output filter
C1
is chosen the value of RC should be
C1
for most applications is 3.3 nF.
30053814
Z(FIL)
30053813
C1
. Can-
P2(EA)
and
,
15
recalculating the value of R
recalculating R
lower crossover frequency. As with any attempt to compen-
sate the LM20146 the stability of the system should be verified
for desired transient droop and settling time.
If the output filter zero, F
quency (F
at the COMP pin to ground. This capacitor adds a pole to
cancel the output filter zero assuring the crossover frequency
will occur before the double pole at f
margin. The output filter zero is set by the output capacitor
value and ESR as shown in the equation below.
If needed, the value for C
equation shown below.
Where R
R
AVIN FILTERING COMPONENTS (C
To prevent high frequency noise spikes from disturbing the
sensitive analog circuitry connected to the AVIN and AGND
pins, a high frequency RC filter is required between PVIN and
AVIN. These components are shown in Figure 3. as C
R
ommended value of C
should be placed as close to the IC as possible with a direct
connection from AVIN to AGND. A good quality X5R or X7R
ceramic capacitor should be used for C
SUB-REGULATOR BYPASS CAPACITOR (C
The capacitor at the VCC pin provides noise filtering and sta-
bility for the internal sub-regulator. The recommended value
of C
10 µF. The capacitor should be a good quality ceramic X5R
or X7R capacitor. In general, a 1 µF ceramic capacitor is rec-
ommended for most applications.
SETTING THE START UP TIME (C
The addition of a capacitor connected from the SS pin to
ground sets the time at which the output voltage will reach the
final regulated value. Larger values for C
start up times. Table 3, shown below provides a list of soft
start capacitors and the corresponding typical start up times.
If different start up times are needed the equation shown be-
low can be used to calculate the start up time.
C1
F
. The required value for R
VCC
is the calculated compensation resistance.
TABLE 3. Start Up Times for Different Soft-Start
Start Up Time (ms)
should be no smaller than 1 µF and no greater than
ESR
C
), an additional capacitor (C
is the output capacitor series resistance and
10
15
20
1
5
C1
will provide additional phase margin at a
F
Capacitors
Z(FIL)
is 1.0 µF. The filter capacitor, C
C2
C1
F
should be calculated using the
. Likewise, increasing C
approaches the crossover fre-
is 1Ω. C
SW
SS
F
/2 degrades the phase
)
F
F
C2
and R
SS
.
must be used. Rec-
C
) should be placed
will result in longer
SS
none
100
120
33
68
(nF)
F
)
VCC
www.national.com
)
C1
F
and
and
F