LM3488MM/NOPB National Semiconductor, LM3488MM/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM3488MM/NOPB
• Description: IC CTRLR SWITCH REG N-CH 8-MSOP
• Manufacturer: National Semiconductor
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LM3488MM/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.26V
• Current - Output: 1A
• Frequency - Switching: 100kHz ~ 1MHz
• Voltage - Input: 2.97 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM3488MM; LM3488MM; LM3488MMTR

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
however, is not a constant value and changes over the course
of a period as a result of the internal compensation ramp (See
Figure 3). Therefore the current limit threshold will also
change. The actual current limit threshold is a function of the
sense voltage (V
Where ISW
equation below. As duty cycle increases, the control voltage
is reduced as V
varies with duty cycle, the following equation should be used
to select R
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. To verify
stability, use the following equation:
If the selected R
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 R
ing equation:
SEN
SL
is used to add additional slope compensation when re-
is part of the current mode control loop and has some
R
SEN
SEN
SEN
LIMIT
x ISW
range. Therefore, the following equation is rec-
and set the desired current limit threshold:
SEN
is the peak switch current limit, defined by the
SENSE
SL
LIMIT
ramps up. Since current limit threshold
is greater than this value, additional slope
) and the internal compensation ramp:
= VCS
SENSE
SL
SENSE
SENSE
ratio value given in the EC table:
MAX
are specified in the electrical
represents the maximum val-
and V
= V
SL
SEN
CS
according to the follow-
SENSE
SL
. This control signal,
. Current limit occurs
are specified in the
CS
- (D x V
, and ISW
SENSE
SL
should be
SL
. When
)
LIMIT
is
17
Where R
R
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
and calculated as:
This changes the equation for current limit (or R
The R
in order to achieve both the desired current limit and stable
operation. In some designs R
on the ISEN pin.
If the inductor is selected such that ripple current is the rec-
ommended 30% value, and the current limit threshold is 120%
of the maximum peak, a simpler method can be used to de-
termine R
bility without RSL, provided that the above 2 conditions are
met:
POWER DIODE SELECTION
Observation of the boost converter circuit shows that the av-
erage current through the diode is the average load current,
and the peak current through the diode is the peak current
through the inductor. The diode should be rated to handle
more than its peak current. The peak diode current can be
calculated using the formula:
In the above equation, I
been defined in
The peak reverse voltage for boost converter is equal to the
regulator output voltage. The diode must be capable of han-
dling this voltage. To improve efficiency, a low forward drop
schottky diode is recommended.
POWER MOSFET SELECTION
The drive pin of LM3488 must be connected to the gate of an
external MOSFET. In a boost topology, the drain of the ex-
ternal N-Channel MOSFET is connected to the inductor and
the source is connected to the ground. The drive pin (DR)
voltage depends on the input voltage (see typical perfor-
mance characteristics). In most applications, a logic level
MOSFET can be used. For very low input voltages, a sub-
logic level MOSFET should be used.
The selected MOSFET directly controls the efficiency. The
critical parameters for selection of a MOSFET are:
1.
SL
installed, the control signal includes additional external
Minimum threshold voltage, V
SEN
SEN
and R
SEN
SL
is the selected value based on current limit. With
is the additional slope compensation generated
V
. The equation below will provide optimum sta-
CS
SL
Figure 11
I
= V
values may have to be calculated iteratively
D(Peak)
ΔV
SENSE
SL
OUT
= I
= 40 µA x R
– (D x (V
OUT
is the output current and ΔI
SL
/ (1−D) + ΔI
can also help to filter noise
TH
SL
(MIN)
SL
+ ΔV
L
SL
))
SEN
www.national.com
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has