LM3478MAX/NOPB National Semiconductor, LM3478MAX/NOPB Datasheet - Page 16

• Manufacturer Part Number: LM3478MAX/NOPB
• Description: IC CTRLR SW REG N-CH 8SOIC
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM3478MAX/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 100%
• Voltage - Supply: 2.97 V ~ 40 V
• Buck: No
• Boost: Yes
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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PROGRAMMING THE OUTPUT VOLTAGE
The output voltage can be programmed using a resistor di-
vider between the output and the FB pin. The resistors are
selected such that the voltage at the FB pin is 1.26V. Pick
R
pin) and R
ground) can be selected using the following equation,
A 100pF capacitor may be connected between the feedback
and ground pins to reduce noise.
SETTING THE CURRENT LIMIT
The maximum amount of current that can be delivered to the
load is set by the sense resistor, R
when the voltage that is generated across the sense resistor
equals the current sense threshold voltage, V
this threshold is reached, the switch will be turned off until the
next cycle. Limits for V
characteristics section. V
ue of the internal control signal V
This control signal, however, is not a constant value and
changes over the course of a period as a result of the internal
compensation ramp (V
old will also change. The actual current limit threshold is a
function of the sense voltage (V
pensation ramp:
Where ISW
equation below.
Figure 10
change with duty cycle. The curve is equivalent to the internal
compensation ramp slope (S
cycle by V
creases, the control voltage is reduced as V
graph also shows the short circuit current limit threshold of
343 mV (typical) during the 325 ns (typical) blanking time. For
higher frequencies this fixed blanking time obviously occupies
more duty cycle, percentage wise. Since current limit thresh-
old varies with duty cycle, the following equation should be
used to select R
F1
FIGURE 10. Current Sense Voltage vs Duty Cycle
(the resistor between the output voltage and the feedback
R
SEN
shows how V
SENSE
F2
LIMIT
x ISW
R
(the resistor between the feedback pin and
F2
SEN
is the peak switch current limit, defined by the
, shown as a dotted line. As duty cycle in-
= (1.26V x R
LIMIT
and set the desired current limit threshold:
SL
CS
= VCS
SENSE
). Therefore the current limit thresh-
SENSE
(and current limit threshold voltage)
MAX
e
F1
are specified in the electrical
) and is bounded at low duty
represents the maximum val-
) / (V
SENSE
= V
CS
SEN
out
as shown in
SENSE
) and the internal com-
. Current limit occurs
- 1.26V)
SL
- (D x V
ramps up. The
SENSE
10135552
Figure
SL
. When
)
10.
16
The numerator of the above equation is V
calculated as:
To avoid false triggering, the current limit value should have
some margin above the maximum operating value, typically
120%. Values for both V
characteristic table. However, calculating with the limits of
these two specs could result in an unrealistically wide current
limit or R
ommended, using the V
R
influence on control loop stability. Therefore, once the current
limit threshold is set, loop stability must be verified. As de-
scribed in the slope compensation section, the following must
hold true for a current mode converter to be stable:
To verify that this equation holds true, use the following equa-
tion:
If the selected RSEN is greater than this value, additional
slope compensation must be added to ensure stability, as
described in the section below.
CURRENT LIMIT WITH EXTERNAL SLOPE
COMPENSATION
R
quired. It is not necessary in most designs and R
no larger than necessary. Select RSL according to the fol-
lowing equation:
Where R
RSL installed, the control signal includes additional external
slope to stabilize the loop, which will also have an effect on
the current limit threshold. Therefore, the current limit thresh-
old must be re-verified, as illustrated in the equations below :
Where ΔV
as discussed in the slope compensation ramp section and
calculated as:
SEN
SL
is used to add additional slope compensation when re-
is part of the current mode control loop and has some
SEN
SEN
SL
is the selected value based on current limit. With
is the additional slope compensation generated
range. Therefore, the following equation is rec-
V
CS
= V
ΔV
SENSE
S
f
SL
SL
- S
SENSE
= 40 µA x R
ratio value given in the EC table:
– (D x (V
e
< S
and V
n
+ S
SL
e
SL
SL
+ ΔV
are specified in the
CS
SL
, and ISW
))
SL
should be
LIMIT
is