SGM3122 Datasheet, PDF (11/17 Pages) SG Micro Corp – PWM Brightness Control

SGM3122
White LED Charge Pump Current
Source with PWM Brightness Control
THEORY OF OPERATION/DESIGN PROCEDURE
Capacitor Selection
Ceramic capacitors such as X5R or X7R are recommended to be used with the SGM3122. For the two flying capacitors
C1 and C2, it is important to use low ESR capacitors to avoid unnecessary efficiency losses. Low ESR capacitors on
VOUT reduce the ripple voltage on the supply of the current sources. Table 3 lists capacitor types that have been tested
with the SGM3122.
Table 3. Capacitors
PART
C1608X5R1A105M
C1608X5R1A474M
C2012X7R1C105M
LMK107BJ105MA
LMK107BJ474MA
LMK212BJ105MG
VALUE
1μF
0.47μF
1μF
1μF
0.47μF
1μF
VOLTAGE
10V
10V
16V
10V
10V
10V
MANUFACTURER
TDK
Taiyo Yuden
SIZE
0603
0603
0805
0603
0603
0805
WEBSITE
www.componnent.tdk.com
www.t-yuden.com
Power Dissipation
The maximum power dissipation inside the SGM3122
can be calculated based on the following equation:
P_{D max} = [(1.5 × V_IN) - V_O + 0.4V] × I_O
The maximum power dissipation occurs when the input
voltage is just low enough to operate in 1.5× mode, with
a forward voltage of the white LED at maximum. This is
typically for V_IN = 4.2V and a forward voltage of 3.6V.
This needs to be lower than the maximum allowed power
dissipation of the package, which can be calculated using
the following equation:
P_{D_MAX, package} =
T_Jmax  T_A
R _θJA
For example, the worst case power dissipation occurs at
the input voltage level where the charge pump switches
from the 1× mode to the 1.5× mode. At this operating
point, the supply voltage to the current sources is at its
maximum and the current sources must drop the most
voltage in order to maintain a regulated output current.
The worst case power dissipation occurs when all 4 LED
outputs are fully loaded with 30mA of LED current.
 With: V_IN = 4.2V, V_f = 3.6V, I_O = 75mA (1.5× mode)
 P_{D_MAX} = 0.203W
Power Efficiency
The power conversion efficiency of the SGM3122 can be
calculated by adding up the products of each LED current
and voltage and dividing it by the product of the input
voltage and current. With a fully charged battery where
the input voltage is typically above the LED forward
voltage, the charge pump operates in the 1x mode and
efficiency is very high. As the battery discharges, there is
a point where the current sources no longer have enough
voltage overhead to maintain a constant current
regulation.
At that point, the charge pump switches into the 1.5×
mode. The conversion efficiency is lowest at the
crossover. As the battery discharges further, the
efficiency again increases until at about 3.2V where it
reaches a second maximum. Below 3.2V input voltage,
the maximum current per LED is less than 30mA.
SG Micro Corp
11
www.sg-micro.com