I'm currently using a linear current source to drive a single LED as in here. In my case (following the notation in the link), VDD is directly connected to a single cell lithium polymer battery, i.e. Vcc = 3.0 ~ 4.2 V. For the transistor and MOSFET, I am using MMBT3904LT3G(Digikey link) and 2N7002P,235(Digikey link), respectively. Rs is set to 2.2 Ohms.

Here is the circuit diagram from the site:

The problem is that there is so much heat dissipated it is hard to touch the PCB board. There are 3 LEDs (and corresponding circuits) in total on a 50 mm x 50 mm PCB (FR-4, 1.6 mm thickness). I am measuring 565 mV (which actually keeps dropping as the PCB heats up) across R2 and a voltage drop of 2.14 V across the LED at Vcc = 4.2 V. I am guessing that most of the heat is generated from Q2, since it has to drop the rest of the voltage which is approximately 1.5 V.

I have considered using a step-down switching regulator to drop the battery voltage to around 3 V, which I certainly think will help the problem. I have also looked on Digikey to find dedicated ICs such as this one. However, both require an inductor that I have heard to be expensive. There also seems to be charge pumps that I am not sure will be effective in my case (LED forward voltage is always lower than the battery voltage).

What would be the best way to go? In the end, I am looking for a cost-effective solution, but now open to any suggestions. And could you clarify whether charge pump LED drivers (such as this) will be effective for my case.

I'm currently using a linear current source to drive a single LED as in here. In my case (following the notation in the link), VDD is directly connected to a single cell lithium polymer battery, i.e. Vcc = 3.0 ~ 4.2 V. For the transistor and MOSFET, I am using MMBT3904LT3G(Digikey link) and 2N7002P,235(Digikey link), respectively. Rs is set to 2.2 Ohms. My desired current is approximately 250 mA.

Here is the circuit diagram from the site:

The problem is that there is so much heat dissipated it is hard to touch the PCB board. There are 3 LEDs (and corresponding circuits) in total on a 50 mm x 50 mm PCB (FR-4, 1.6 mm thickness). I am measuring 565 mV (which actually keeps dropping as the PCB heats up) across R2 and a voltage drop of 2.14 V across the LED at Vcc = 4.2 V. I am guessing that most of the heat is generated from Q2, since it has to drop the rest of the voltage which is approximately 1.5 V.

I have considered using a step-down switching regulator to drop the battery voltage to around 3 V, which I certainly think will help the problem. I have also looked on Digikey to find dedicated ICs such as this one. However, both require an inductor that I have heard to be expensive. There also seems to be charge pumps that I am not sure will be effective in my case (LED forward voltage is always lower than the battery voltage).

What would be the best way to go? In the end, I am looking for a cost-effective solution, but now open to any suggestions. And could you clarify whether charge pump LED drivers (such as this) will be effective for my case.

I'm currently using a linear current source to drive a single LED as in the answer of How can I efficiently drive an LED?. In my case (following the notation in the link), Vcc is directly connected to a single cell lithium polymer battery, i.e. Vcc = 3.0 ~ 4.2 V. For Q1 and Q2, I am using MMBT3904LT3G(Digikey link) and 2N7002P,235(Digikey link), respectively. R2 is set to 2.2 Ohms.

The problem is that there is so much heat dissipated it is hard to touch the PCB board. There are 3 LEDs (and corresponding circuits) in total on a 50 mm x 50 mm PCB (FR-4, 1.6 mm thickness). I am measuring 565 mV (which actually keeps dropping as the PCB heats up) across R2 and a voltage drop of 2.14 V across the LED at Vcc = 4.2 V. I am guessing that most of the heat is generated from Q2, since it has to drop the rest of the voltage which is approximately 1.5 V.

I have considered using a step-down switching regulator to drop the battery voltage to around 3 V, which I certainly think will help the problem. I have also looked on Digikey to find dedicated ICs such as this one. However, both require an inductor that I have heard to be expensive. There also seems to be charge pumps that I am not sure will be effective in my case (LED forward voltage is always lower than the battery voltage).

What would be the best way to go? In the end, I am looking for a cost-effective solution, but now open to any suggestions. And could you clarify whether charge pump LED drivers (such as this) will be effective for my case.

I'm currently using a linear current source to drive a single LED as in here. In my case (following the notation in the link), VDD is directly connected to a single cell lithium polymer battery, i.e. Vcc = 3.0 ~ 4.2 V. For the transistor and MOSFET, I am using MMBT3904LT3G(Digikey link) and 2N7002P,235(Digikey link), respectively. Rs is set to 2.2 Ohms.

Here is the circuit diagram from the site:

The problem is that there is so much heat dissipated it is hard to touch the PCB board. There are 3 LEDs (and corresponding circuits) in total on a 50 mm x 50 mm PCB (FR-4, 1.6 mm thickness). I am measuring 565 mV (which actually keeps dropping as the PCB heats up) across R2 and a voltage drop of 2.14 V across the LED at Vcc = 4.2 V. I am guessing that most of the heat is generated from Q2, since it has to drop the rest of the voltage which is approximately 1.5 V.

I have considered using a step-down switching regulator to drop the battery voltage to around 3 V, which I certainly think will help the problem. I have also looked on Digikey to find dedicated ICs such as this one. However, both require an inductor that I have heard to be expensive. There also seems to be charge pumps that I am not sure will be effective in my case (LED forward voltage is always lower than the battery voltage).

What would be the best way to go? In the end, I am looking for a cost-effective solution, but now open to any suggestions. And could you clarify whether charge pump LED drivers (such as this) will be effective for my case.