LM3450MT/NOPB National Semiconductor, LM3450MT/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM3450MT/NOPB
• Description: IC LED DRIVER DIMMING 16-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3450MT/NOPB

• Featured Product: LM3450 LED Driver
• Internal Driver: No
• Type - Primary: General Purpose
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: PWM
• Number Of Outputs: 1
• Voltage - Supply: 8.5 V ~ 20 V
• Voltage - Output: 11.5V
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Operating Temperature: -40°C ~ 125°C
• Led Driver Application: General Lighting
• No. Of Outputs: 1
• Output Current: 200mA
• Output Voltage: 11.5V
• Input Voltage: 8.5V To 20V
• Dimming Control Type: PWM / Analog
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Frequency: -

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POWER FACTOR CONTROLLER
The LM3450 uses CRM control to regulate the output voltage
and provide power factor correction. In a non-isolated boost
topology, an external voltage divider (R
sense the output voltage, as shown in
is connected to the inverting input (FB) of the internal error
amplifier. The LM3450 regulates the feedback voltage (V
to 2.5V in a closed loop fashion.
The FB pin has a shutdown mode to protect against a feed-
back short which can cause excessive energy transfer to the
output. In the same manner, the FB pin also has an OVP
mode which terminates switching when output over-voltage
is sensed.
With the FB shutdown mode, it is necessary to have a pre-
liminary biasing method for the output of the error amplifier
(COMP). Otherwise, the converter would never start. COMP
is pre-biased with a 415µA current until the voltage at COMP
(V
For an isolated topology flyback topology, where the error
amplifier is on the secondary, the LM3450 internal error am-
plifier can be bypassed engaging a 5kΩ pull-up resistor at
COMP. COMP can then be connected directly to the optical
isolation as shown in
COMP and the sensed rectified AC input voltage (V
vided via a resistor divider (R
multiplier. The current through the sense resistor (R
duces a voltage (V
put. When V
off. The peak detect threshold and the current slope during
an on-time are proportionally changing which yields a nearly
constant on-time, shown in
CMP
) exceeds the minimum operational voltage (V
CS
exceeds the multiplier output, Q
CS
) that is compared to the multiplier out-
Figure
Figure
5.
AC1
, RAC2), are inputs to the
4:
Figure
FB1
, R
FB2
5. The divider
FIGURE 5. PFC Control Circuit
SW
) is used to
is turned
THM
AC
CS
), pro-
) pro-
).
FB
)
14
Once Q
(boost) or transformer (flyback) is demagnetized to turn Q
on again. Demagnetization, sensed at ZCD, occurs when the
current through the magnetic component falls to zero. Since
the output voltage is regulated, the slope of the current re-
mains relatively constant and, coupled with the variable peak
detect, creates a variable off-time.
The sinusoidal peak detection envelope creates an input cur-
rent that is sinusoidal and in phase with the input voltage
providing excellent PF. The PWM comparator 30mV input
offset voltage ensures current is also drawn at the zero-cross-
ings of the rectified AC line, reducing distortion and further
improving PF.
CURRENT SENSE
The LM3450 senses current through Q
(R
ceeds the output of the multiplier (V
V
scaled rectified AC voltage (V
enced from its operational minimum (V
tiplier gain (K
It should be noted that the LM3450 leading edge blanking
(LEB) circuit pulls the current sense input to the PWM com-
parator low for 140µs at the beginning of each on-time. The
LEB blanks the current spike and associated ringing due to
the turn-on transient of Q
duty cycle.
OVER CURRENT PROTECTION
The LM3450 has a current limit threshold (V
to protect Q
current conditions. If V
turned off.
MLT
CS
) between the source of Q
is variable over the line cycle and is a function of the
SW
is turned off, the LM3450 waits until the inductor
SW
M
) and the PWM comparator offset (V
along with the rest of the system from over-
CS
SW
exceeds V
, limiting the minimum achievable
AC
SW
), the COMP voltage refer-
and GND. When V
LIM
MLT
SW
COMP
, Q
), Q
via a sense resistor
SW
LIM
SW
-V
is immediately
THM
= 1.5V) at CS
30127426
is turned off.
), the mul-
OS
):
CS
ex-
SW