LM3409MY-NOPB Datasheet, PDF (17/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

LM3409, LM3409HV, LM3409-Q1
www.ti.com
SNVS602J – MARCH 2009 – REVISED MAY 2013
OUTPUT OVER-VOLTAGE PROTECTION
Because the LM3409/09HV controls a buck current regulator, there is no inherent need to provide output over-
voltage protection. If the LED load is opened, the output voltage will only rise as high as the input voltage plus
any ringing due to the parasitic inductance and capacitance present at the output node. If a ceramic output
capacitor is used in the application, it should have a minimum rating equal to the input voltage. Ringing seen at
the output node should not damage most ceramic capacitors, due to their high ripple current rating.
INPUT CAPACITORS
Input capacitors are selected using requirements for minimum capacitance and RMS ripple current. The PFET
current during t_ON is approximately I_LED, therefore the input capacitors discharge the difference between I_LED and
the average input current (I_IN) during t_ON. During t_OFF, the input voltage source charges up the input capacitors
with I_IN. The minimum input capacitance (C_IN-MIN) is selected using the maximum input voltage ripple (Δv_IN-MAX)
which can be tolerated. Δv_IN-MAX is equal to the change in voltage across C_IN during t_ON when it supplies the load
current. A good starting point for selection of C_IN is to use Δv_IN-MAX of 2% to 10% of V_IN. C_IN-MIN can be selected
as follows:
C_{IN- MIN}
=
I_LED x t_ON
'v_{IN- MAX}
=
I_LED
x _{¨¨©§fS}¹_W - t_{OFF¸¸¹·}
'v_{IN- MAX}
(20)
An input capacitance at least 75% greater than the calculated C_IN-MIN value is recommended. To determine the
RMS input current rating (I_IN-RMS) the following approximation can be used:
I_IN-RMS = I_LED x D x (1 - D) = I_LED x f_SW x t_ON x t_OFF
(21)
Since this approximation assumes there is no inductor ripple current, the value should be increased by 10-30%
depending on the amount of ripple that is expected. Ceramic capacitors are the best choice for input capacitors
for the same reasons mentioned in the BUCK CONVERTERS WITH OUTPUT CAPACITORS section. Careful
selection of the capacitor requires checking capacitance ratings at the nominal operating voltage and
temperature.
P-CHANNEL MosFET (PFET)
The LM3409/09HV requires an external PFET (Q1) as the main power MosFET for the switching regulator. Q1
should have a voltage rating at least 15% higher than the maximum input voltage to ensure safe operation during
the ringing of the switch node. In practice all switching converters have some ringing at the switch node due to
the diode parasitic capacitance and the lead inductance. The PFET should also have a current rating at least
10% higher than the average transistor current (I_T):
I_T = D x I_LED
(22)
The power rating is verified by calculating the power loss (P_T) using the RMS transistor current (I_T-RMS) and the
PFET on-resistance (R_DS-ON):
I_T-RMS = I_LED x
D
x
¨¨¨©§¹⁺
1
12
x
¨¨©§
'_{IiLLE-DPP¸¸¹·}²
¸·
¸¸¹
(23)
P_T
=
I_T-
2
RMS
x
R_DSON
(24)
It is important to consider the gate charge of Q1. As the input voltage increases from a nominal voltage to its
maximum input voltage, the COFT architecture will naturally increase the switching frequency. The dominant
switching losses are determined by input voltage, switching frequency, and PFET total gate charge (Q_g). The
LM3409/09HV has to provide and remove charge Q_g from the input capacitance of Q1 in order to turn it on and
off. This occurs more often at higher switching frequencies which requires more current from the internal
regulator, thereby increasing internal power dissipation and eventually causing the LM3409/09HV to thermally
cycle. For a given range of operating points the only effective way to reduce these switching losses is to
minimize Q_g.
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