LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 17

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

CURRENT LIMIT WITH ADDITIONAL SLOPE
COMPENSATION
If an external slope compensation resistor is used (see Figure
4) the internal control signal will be modified and this will have
an effect on the current limit. The control signal is given by:
Where V
cal characteristics section. If R
the existing slope compensation. The command voltage will
then be given by:
Where ΔV
and can be calculated by use of Figure 5 or is equal to 40 x
10
Therefore R
for setting the current limit.
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
−6
* R
SL
SENSE
. This changes the equation for R
SL
SL
V
is the additional slope compensation generated
CS
and V
can be used to provide an additional method
= V
V
CS
SENSE
SL
= V
are defined parameters in the electri-
SENSE
− (D * ( V
SL
− (D * V
is used, then this will add to
SL
+ ΔV
SL
)
FIGURE 12. Adjusting the Output Voltage
SEN
SL
) )
to:
17
more than its peak current. The peak diode current can be
calculated using the formula:
In the above equation, I
been defined in Figure 11
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.
2.
3.
4.
5.
The off-state voltage of the MOSFET is approximately equal
to the output voltage. V
greater than the output voltage. The power losses in the
Minimum threshold voltage, V
On-resistance, R
Total gate charge, Q
Reverse transfer capacitance, C
Maximum drain to source voltage, V
I
D(Peak)
DS
OUT
(ON)
= I
g
DS(MAX)
OUT
is the output current and ΔI
/ (1−D) + ΔI
10138820
of the MOSFET must be
TH
(MIN)
RSS
DS(MAX)
L
www.national.com
L
has