Rainbow Electronics MAX16831 - Manual (Page 15)

When using the MAX16831 in a boost or buck-boost
configuration, the input RMS current is low and the
input capacitance can be small.
Operating the MAX16831 Without the
Dimming Switch
The MAX16831 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 MAX16831 can still be used.
A short circuit across the load will cause the MAX16831
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.
MOSFET losses may be broken into three categories:
conduction loss, gate drive loss, and switching loss.
The following simplified power loss equation is true for
all the different configurations.
P
LOSS
= P
CONDUCTION
+ P
GATEDRIVE
+ P
SWITCH
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 minimize current loops. Use ground planes for best
results.
Careful PCB layout is critical to achieve low switching
losses and clean, stable operation. Use a multilayer
board whenever possible for better noise performance
and power dissipation. Follow these guidelines for
good PCB layout:
• Use a large copper plane under the MAX16831
package. Ensure that all heat-dissipating compo-
nents have adequate cooling. Connect the exposed
pad of the device to the ground plane.
• Isolate the power components and high-current paths
from sensitive analog circuitry.
• Keep the high-current paths short, especially at the
ground terminals. This practice is essential for sta-
ble, jitter-free operation. Keep switching loops short.
• Connect AGND, SGND, and QGND to a ground
plane. Ensure a low-impedance connection between
all ground points.
• Keep the power traces and load connections short.
This practice is essential for high efficiency. Use
thick copper PCBs (2oz vs. 1oz) to enhance full-load
efficiency.
• Ensure that the feedback connection to FB is short
and direct.
• Route high-speed switching nodes away from the
sensitive analog areas.
• To prevent discharge of the compensation capaci-
tors, C1 and C2, during the off-time of the dimming
cycle, ensure that the PCB area close to these com-
ponents has extremely low leakage. Discharge of
these capacitors due to leakage may result in
degraded dimming performance.
Chip Information
PROCESS: BICMOS
MAX16831
High-Voltage, High-Power LED Driver with
Analog and PWM Dimming Control
______________________________________________________________________________________ 15
32 31 30 29 28 27 26
9 10 11 12 13 14 15
18
19
20
21
22
23
24
7
6
5
4
3
2
1
MAX16831
TQFN
(5mm x 5mm)
TOP VIEW
UVEN
N.C.
REG1
AGND
REF
DIM
RTSYNC
8 CLKOUT
I.C. V
CC
REG2 HI CLMP CS- CS+
25
LO
N.C.
DGT
QGND
SNS-
SNS+
DRI
DRV
17 SGND
OV FB
16
SGND
CS
COMP
I.C. I.C. I.C.
*EP
+
*EP = EXPOSED PAD
Pin Configuration