Datasheet

3 Design Procedure Divider Network
R1
1
V
0.898 V
R = = = 898
0.1 mA 0.1 mA
W
R2
2
V
2.147 V
R = = = 21.47
0.1 mA 0.1 m A
W
Design Procedure Divider Network www.ti.com
The 3.3-V (U2) section of the EVM is configured to operate with an input voltage from 10.5 V to 17 V. EN
jumper JP2 must be installed in the desired ON or OFF position. The unit operates with the SYNC jumper
PWM or PWM/PFM position; this impacts efficiency. See the data sheet for additional information
The TPS62113 feedback voltage (FB) is set to 1.153 V. If this is used for current regulation, the power
dissipation of the sense resistor is high. To reduce the power dissipation on the current sense resistor R
10
,
a reference voltage of 3.3 V is summed resulting in the reduction of the V
sense
voltage from 1.153 V to
0.255 V.
The first step is to choose the LED operating current and V
sense
voltage which will determine the value for
R
10
. A tradeoff occurs, however, between the power dissipation on R
10
and the accuracy of the regulation
point. As V
sense
is increased, the current regulation accuracy improves; the maximum voltage is 1.153 V.
As V
sense
is decreased, the power dissipation is reduced, but error due to V
ref
, resistors R
1
and R
2
increases.
The EVM I
LED
is set to 500 mA with a V
sense
voltage of 0.225 V.
V
sense
= I
LED
× R
10
= 500 mA × 510 m Ω = 0.255 V
V
R1
= V
FB
– V
sense
= 1.153 V – 0.255 V = 0.898 V
V
R2
= V
REF
– V
FB
= 3.3 V – 1.153 V = 2.147 V
Choose divider current of 0.1 mA:
Standard value 8.87 k Ω :
Standard value 21.5 k Ω .
TPS62113EVM-465 4 SLVU321 – September 2009
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