LM3409MY-NOPB Datasheet, PDF (18/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

LM3409, LM3409HV, LM3409-Q1
SNVS602J – MARCH 2009 – REVISED MAY 2013
www.ti.com
A good rule of thumb is to limit Q_g < 30nC (if the switching frequency remains below 300kHz for the entire
operating range then a larger Q_g can be considered). If a PFET with small R_DS-ON and a high voltage rating is
required, there may be no choice but to use a PFET with Q_g > 30nC.
When using a PFET with Q_g > 30nC, the bypass capacitor (C_F) should not be connected to the VIN pin. This will
ensure that peak current detection through R_SNS is not affected by the charging of the PFET input capacitance
during switching, which can cause false triggering of the peak detection comparator. Instead, C_F should be
connected from the VCC pin to the CSN pin which will cause a small DC offset in V_CST and ultimately I_LED,
however it avoids the problematic false triggering.
In general, the PFET should be chosen to meet the Q_g specification whenever possible, while minimizing R_DS-ON.
This will minimize power losses while ensuring the part functions correctly over the full operating range.
RE-CIRCULATING DIODE
A re-circulating diode (D1) is required to carry the inductor current during t_OFF. The most efficient choice for D1 is
a Schottky diode due to low forward voltage drop and near-zero reverse recovery time. Similar to Q1, D1 must
have a voltage rating at least 15% higher than the maximum input voltage to ensure safe operation during the
ringing of the switch node and a current rating at least 10% higher than the average diode current (I_D):
I_D = (1- D) x I_LED
(25)
The power rating is verified by calculating the power loss through the diode. This is accomplished by checking
the typical diode forward voltage (V_D) from the I-V curve on the product datasheet and calculating as follows:
P_D = I_D x V_D
(26)
In general, higher current diodes have a lower V_D and come in better performing packages minimizing both
power losses and temperature rise.
i_LED (t)
I_LED-MAX
I_LED
I_DIM-LED
D_DIM x T_DIM
0
T_DIM
t
t_OFF
Figure 30. Ideal LED Current i_LED(t) During Parallel FET Dimming
EXTERNAL PARALLEL FET PWM DIMMING
Any buck topology LED driver is a good candidate for parallel FET dimming because high slew rates are
achievable, due to the fact that no output capacitance is required. This allows for much higher dimming
frequencies than are achievable using the EN pin. When using external parallel FET dimming, a situation can
arise where maximum off-time occurs due to a shorted output. To mitigate this situation, capacitive coupling to
the enable pin can be employed.
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