Download Extending the input voltage range of the

Application Report
SLVA338 – August 2009
Extending the Input Voltage Range of the
TPS6116x/7x/8x/9x WLED Drivers
Jeff Falin.........................................................................................................................................
ABSTRACT
The TPS6116x, TPS6118x and TPS6119x WLED driver and TPS6117x boost
converter integrated circuits (IC) can operate with different input voltages, one powering
the IC itself and the other powering the boost power stage. This application report
explains various options on how to use the WLED drivers with split power rails. This
allows these LED drivers to be used in applications where the available power rail
exceeds the IC's maximum input voltage.
Figure 1 shows a block diagram of the various options available for powering the
WLED drivers, in order by increasing cost.
VPWR
IQ
Existing system
rail < VINmax
VIN
C1a
Option A
IQ
VPWR
D1B
DnB
R1B
VIN
C1b
Option B
IQ
VPWR
VIN
R1C
+
VZ
-
D1C
IZ
C1C
Option C
VIN
Q1
VPWR
R1D
IQ/hFE
D1D
+
VZ
-
WLED Driver
or Boost
Converter IC
IQ
VIN
C2D
IZ
C1D
Option D
Linear Reg
VIN_LDO VO_LDO
VPWR
C1E
IQ
VIN
C2E
Option E
Figure 1. Options for Powering the WLED Drivers From Split Rails
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Option A is simple. If the system has an existing power rail (e.g., VCC = 3.3 V or 5 V) that can provide the
bias power for the IC, then use VCC to provide the IC’s input power. The following table gives the VIN
range and maximum quiescent current, IQmax, for each family of ICs. C1x must be at least 1 µF.
IC FAMILY
VINmin (V)
VINmax (V)
IQmax (mA)
TPS6116x/7x
3
18
2.3
TPS61180/1/2
5
24
3.0
The remaining options assume that the only available power rail, VPWR, may exceed the IC’s maximum
operating voltage. Therefore, assuming VPWRmin > VINmin, either a PN diode, Zener diode (D1), or linear
regulator steps down VPWR to a voltage low enough to power the IC’s input voltage, VIN.
Option B uses multiple series diodes to drop VPWR lower than VINmax. The diodes selected must have
forward voltage drops, VFWD, at the current level, IFWD, such that
VPWRmax - n ´ VFWDmin@I
£ VINmax
FWD
(1)
Because the minimum operating current into the LED driver, IQmin, is not specified and because the current
into VIN during shutdown, ISD, is small, set IFWD using R1B = VINmax/IFWD so that the diodes drop a
minimum forward voltage, VFWDmin.
Option C uses a Zener diode to clamp the voltage at VIN lower than VINmax. The designer must select a
Zener diode such that
VZmax < VINmax and VZmin > VINmin
(2)
Where VZ is the Zener breakdown voltage. It is recommended to select a diode with VZmax no more than
10% below VINmax to minimize the power lost through the Zener diode and resistor R1C. Resistor R1C is
sized so that it is small enough to provide both the IC’s maximum quiescent current, IQmax, and Zener
diode current, IZ, to keep the Zener diode in breakdown, as expressed in Equation 3.
V
- VZmax
V
- 0.9 ´ VINmax
R1C < PWRmax
= PWRmax
IZ + IQmax
IZ + IQmax
(3)
C1B must be at least a 1-µF, ceramic capacitor.
The voltage clamped by the Zener diode varies inversely as IQ varies and directly as VPWR varies, i.e., the
clamped voltage begins to drop as IQ increases because IZ decreases or as VPWR drops. Also, if VPWRmax
is significantly larger than VINmax, the power dissipated through R1C, computed as PD_R1c = (VPWRmax –
VZ)2/R1C may require R1C to be a costly, large footprint resistor. Option D may be a better solution.
Option D shows a discrete linear regulator created by a Zener diode and NPN bipolar transistor. This
circuit reduces the current through the resistor from IZ + IQmax to IZ + IQmax/ hFE, where hFE is the bipolar
transistor’s current gain. Also, the bipolar transistor’s current gain reduces the variation of the voltage at
the IC's VIN pin as IQ changes. Option D requires a Zener diode with slightly larger breakdown voltage:
VZmax < VINmax and VZmin > VINmin + VBEmax
(4)
where VBEmax is the NPN transistor’s maximum base-emitter voltage at IQmax, typically near 0.7 V.
Following the same recommendation in Option C, Equation 2 shows how to compute R1D with VZmax
0.9*VINmax.
- VZmax VPWRmax - 0.9 ´ VINmax
V
R1D < PWRmax
=
IQmax
I
IZ +
IZ + Qmax
hFE
hFE
(5)
C1D must be at least 0.01 µF and C2D must be at least 1 µF.
Option E is to select a linear regulator IC with:
• Wide enough input voltage range to accommodate VPWR
2
Extending the Input Voltage Range of the TPS6116x/7x/8x/9x WLED Drivers
SLVA338 – August 2009
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•
•
Adjustable output voltage VINmin< VO_LDO < VINmax and
PD_LDO > IQmax × (VPWRmax – VO_LDO) where PD_LDO is the LDO package’s maximum power dissipation
rating at the application’s maximum ambient temperature.
C1E must be sized according to the regulator’s data sheet. C2D must be the larger of the linear regulator’s
minimum required output capacitor or 1 µF. TPS71501 is a good choice.
Option B Example
Specification: TPS61165 powered by a power supply with 20 V maximum
Solution: Although its voltage rating is significantly above the minimum for this application, the low-cost
1N4148, 100-mA diode is a good choice.
Per the data sheet, the 1N4148 needs at least 1 mA to drop 0.5 V over TA = –40°C to 75°C. Therefore,
R1B = 18 V/1 mA = 18 kΩ. Solving Equation 1 for n gives n ≥ (VPWR-VINmax / VFWDmin@IFWD = (20 V–18
V)/0.5 V = 4. As a safeguard, five 1N4148s are recommended with R1B = 18 kΩ are recommended.
Option D Example
Specification: IC: TPS61181 powered from a supply with 26 V maximum
Solution: Using Equation 4, the application requires VZmax < 24 V and VZmin ≥ 4.5 V + 0.7 V. Keeping in
mind the 90% recommendation, choose a 0.9 × 24 V = 21.6→ 21 V Zener diode. The MMSZ4707T1 diode
gives 19.0 V < VZ < 21.0 V. The BC848 30-V NPN transistor with hFEmin = 200 is a low-cost generic NPN
transistor. Other NPN transistors are acceptable if their VCE voltage rating is higher than VPWRmax.
Using Equation 5, R2 is sized for the current:
- 0.9 ´ VINmax 26 V - 0.9 × 24 V
V
R1D < PWRmax
=
= 4.35 kW ® 4.32 k W
IQmax
2.3 mA
1
mA
+
IZ +
200
hFE
(6)
Power dissipation in the diode is computed in the following equation:
PDmax =
(VPW Rm ax - VZm in ) × VZmin
R1D
=
(26V - 18.05 V) ´ 18.05 V
= 33 m W
4.32 kW
(7)
This power dissipation is well within the capabilities of the diode’s SOD-123 package.
SLVA338 – August 2009
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