MAX16816 Datasheet, PDF (23/33 Pages) Maxim Integrated Products – Programmable Switch-Mode LED Driver with Analog-Controlled PWM Dimming

Programmable Switch-Mode LED Driver
with Analog-Controlled PWM Dimming
Boost configuration: In the boost converter, the average
inductor current varies with line and the maximum aver-
age current occurs at low line. For the boost converter,
the average inductor current is equal to the input cur-
rent. In this case the inductance, L, is calculated as:
( ) L = VINMIN x VOUT − VINMIN
V_OUT x f_SW x ΔI_L
where V_INMIN is the minimum input voltage, V_OUT is the
output voltage, and f_SW is the switching frequency.
Buck-boost configuration: In a buck-boost converter
the average inductor current is equal to the sum of the
input current and the load current. In this case the
inductance, L, is:
( ) ^{L =}
V_OUT x V_INMIN
V_OUT + V_INMIN x f_SW x ΔI_L
where V_INMIN is the minimum input voltage, V_OUT is the
output voltage, and f_SW is the switching frequency.
Output Capacitor
The function of the output capacitor is to reduce the
output ripple to acceptable levels. The ESR, ESL, and
the bulk capacitance of the output capacitor contribute
to the output ripple. In most of the applications, the out-
put ESR and ESL effects can be dramatically reduced
by using low-ESR ceramic capacitors. To reduce the
ESL effects, connect multiple ceramic capacitors in
parallel to achieve the required bulk capacitance.
In a buck configuration, the output capacitance, C_F, is
calculated using the following equation:
C_F
≥
(V_INMAX − V_OUT) × V_OUT
ΔV_R × 2 × L × V_INMAX × f_SW2
where ΔV_R is the maximum allowable output ripple.
In a boost configuration, the output capacitance, C_F, is
calculated as:
C_F
≥
(V_OUT − V_INMIN) × 2 × I_OUT
ΔV_R × V_OUT × f_SW
where I_OUT is the output current.
In a buck-boost configuration, the output capacitance,
C_F, is calculated as:
C_F
≥
ΔV_R
2 × V_OUT × I_OUT
× (V_OUT + V_INMIN) × f_SW
where V_OUT is the voltage across the load and I_OUT is
the output current.
Input Capacitor
An input capacitor connected between V_CC and
ground must be used when configuring the MAX16816
as a buck converter. Use a low-ESR input capacitor
that can handle the maximum input RMS ripple current.
Calculate the maximum allowable RMS ripple using the
following equation:
I_IN(RMS) = ^{IOUT ×}
V_OUT × (V_INMIN - V_OUT)
V_INMIN
In most of the cases, an additional electrolytic capaci-
tor should be added to prevent input oscillations due to
line impedances.
When using the MAX16816 in a boost or buck-boost
configuration, the input RMS current is low and the
input capacitance can be small (see the Typical
Operating Circuits).
Operating the MAX16816 Without the
Dimming Switch
The MAX16816 can also be used in the absence of the
dimming MOSFET. In this case, the PWM dimming per-
formance is compromised but in applications that do
not require dimming the MAX16816 can still be used. A
short circuit across the load will cause the MAX16816
to disable the gate drivers and they will remain off until
the input power is recycled.
Switching Power MOSFET Losses
When selecting MOSFETs for switching, consider the
total gate charge, power dissipation, the maximum
drain-to-source voltage, and package thermal imped-
ance. The product of the MOSFET gate charge and
R_DS(ON) is a figure of merit, with a lower number signi-
fying better performance. Select MOSFETs optimized
for high-frequency switching applications.
Layout Recommendations
Typically, there are two sources of noise emission in a
switching power supply: high di/dt loops and high dv/dt
surfaces. For example, traces that carry the drain cur-
rent often form high di/dt loops. Similarly, the heatsink
of the MOSFET connected to the device drain presents a
high dv/dt source; therefore, minimize the surface area of
the heatsink as much as possible. Keep all PCB traces
carrying switching currents as short as possible to mini-
mize current loops. Use ground planes for best results.
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