Datasheet

'V
samp1
= -
M
2
M
1
D
1-D
'V
samp0
'V
samp0
= -
PWM
Comparator
Oscillator Sets
the RS Latch
PWM Comparator resets
the RS latch
Blank-Out prevents false
reset
+
_
V
SL
T
min
(on) Blank-Out time
LM3481
www.ti.com SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
Figure 18. Basic Operation of the PWM Comparator
OVER VOLTAGE PROTECTION
The LM3481 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (FB). If
at anytime the voltage at the feedback pin rises to V
FB
+ V
OVP
, OVP is triggered. See the electrical characteristics
section for limits on V
FB
and V
OVP
.
OVP will cause the drive pin (DR) to go low, forcing the power MOSFET off. With the MOSFET off, the output
voltage will drop. The LM3481 will begin switching again when the feedback voltage reaches V
FB
+ (V
OVP
-
V
OVP(HYS)
). See the electrical characteristics section for limits on V
OVP(HYS)
.
The internal bias of the LM3481 comes from either the internal bias voltage generator as shown in the block
diagram or directly from the voltage at the VIN pin. At input voltages lower than 6V the internal IC bias is the
input voltage and at voltages above 6V the internal bias voltage generator of the LM3481 provides the bias.
SLOPE COMPENSATION RAMP
The LM3481 uses a current mode control scheme. The main advantages of current mode control are inherent
cycle-by-cycle current limit for the switch and simpler control loop characteristics. It is easy to parallel power
stages using current mode control since current sharing is automatic. However there is a natural instability that
will occur for duty cycles, D, greater than 50% if additional slope compensation is not addressed as described
below.
The current mode control scheme samples the inductor current, I
L
, and compares the sampled signal, V
samp
, to a
internally generated control signal, V
c
. The current sense resistor, R
SEN
, as shown in Figure 22, converts the
sampled inductor current, I
L
, to the voltage signal, V
samp
, that is proportional to I
L
such that:
V
samp
= I
L
x R
SEN
The rising and falling slopes, M
1
and −M
2
respectively, of V
samp
are also proportional to the inductor current rising
and falling slopes, M
on
and −M
off
respectively. Where M
on
is the inductor slope during the switch on-time and
−M
off
is the inductor slope during the switch off-time and are related to M
1
and −M
2
by:
M
1
= M
on
x R
SEN
−M
2
= −M
off
x R
SEN
For the boost topology:
M
on
= V
IN
/ L
−M
off
= (V
IN
− V
OUT
) / L
M
1
= [V
IN
/ L] x R
SEN
−M
2
= [(V
IN
− V
OUT
) / L] x R
SEN
M
2
= [(V
OUT
− V
IN
) / L] x R
SEN
Current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 19, where
the control signal slope, M
C
, equals zero. In Figure 19, a small increase in the load current causes the sampled
signal to increase by ΔV
samp0
. The effect of this load change, ΔV
samp1
, at the end of the first switching cycle is :
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