MAX16834_10 Datasheet, PDF (15/23 Pages) Maxim Integrated Products – High-Power LED Driver with Integrated High-Side LED Current Sense and PWM Dimming MOSFET Driver

High-Power LED Driver with Integrated High-Side LED
Current Sense and PWM Dimming MOSFET Driver
lowing equation. The MOSFET must be mounted on a
board as per manufacturer specifications to dissipate
the heat.
The RMS current rating of the switching MOSFET Q1 is
calculated as follows for boost and boost-buck configu-
rations:
ID_RMS
=
⎛
⎝⎜
(IL _AVG )²
×
D_MAX
⎞
⎠⎟
× 1.3
where ID_RMS is the MOSFET Q1’s drain RMS current in
amperes.
The MOSFET Q1 will dissipate power due to both
switching losses as well as conduction losses. The con-
duction losses in the MOSFET is calculated as follows:
P_COND = (IL _AVG )² × D_MAX × R_DSON
where R_DSON is the on-resistance of Q1 in ohms with
an assumed junction temperature of +100°C, P_COND is
in watts, and IL_AVG is in amperes.
Use the following equations to calculate the switching
losses in the MOSFET:
Boost configuration:
P_SW
=
⎛
⎜
IL
AVG
⎝
×
V_LED ²
2
×
C_GD
×
f_SW
⎞
⎟
⎠
×
⎛1
⎝⎜ IG_ON
+
1
IG_OFF
⎞
⎠⎟
Boost-buck configuration:
P_SW
=
⎛
⎜
IL
AVG
⎝
× (V_LED
+ V_INMAX )²
2
× C_GD
× f_SW
⎞
⎟
⎠
×
⎛1
⎝⎜ IG_ON
+
1
IG_OFF
⎞
⎠⎟
where IG_ON and IG_OFF are the gate currents of the
MOSFET Q1 in amperes when it is turned on and
turned off, respectively, V_LED and V_INMAX are in volts,
IL_AVG is in amperes, f_SW is in hertz, and C_GD is the
gate-to-drain MOSFET capacitance in farads.
Choose a MOSFET that has a higher power rating than
that calculated by the following equation when the
MOSFET case temperature is at +70°C:
P_TOT (W) = P_COND(W) + P_SW (W)
Rectifier Diode
Use a Schottky diode as the rectifier (D1) for fast
switching and to reduce power dissipation. The select-
ed Schottky diode must have a voltage rating 20%
above the maximum converter output voltage. The max-
imum converter output voltage is V_LED in boost configu-
ration and V_LED + V_INMAX in boost-buck configuration.
The current rating of the diode should be greater than
I_D in the following equation:
I_D = IL _AVG × (1- D_MAX ) × 1.5
Dimming MOSFET
Select a dimming MOSFET (Q2) with continuous current
rating at +70°C, higher than the LED current by 30%.
The drain-to-source voltage rating of the dimming
MOSFET must be higher than V_LED by 20%.
Feedback Compensation
The LED current control loop comprising of the switch-
ing converter, the LED current amplifier, and the error
amplifier should be compensated for stable control of
the LED current. The switching converter small-signal
transfer function has a right half-plane (RHP) zero for
both boost and boost-buck configurations as the induc-
tor current is in continuous conduction mode. The RHP
zero adds a 20dB/decade gain together with a 90°
phase lag, which is difficult to compensate. The easiest
way to avoid this zero is to roll off the loop gain to 0dB
at a frequency less than one-fifth of the RHP zero fre-
quency with a -20dB/decade slope.
The worst-case RHP zero frequency (f_ZRHP) is calculat-
ed as follows:
Boost configuration:
f_ZRHP =
V_LED × (1- D_MAX )²
2π × L × I_LED
Boost-buck configuration:
f_ZRHP =
V_LED × (1- D_MAX )²
2π × L × I_LED × D_MAX
where f_ZRHP is in hertz, V_LED is in volts, L is the induc-
tance value of L1 in henries (H), and I_LED is in amperes.
The switching converter small-signal transfer function
also has an output pole for both boost and boost-buck
configurations. The effective output impedance that
determines the output pole frequency together with the
output filter capacitance is calculated as:
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