ST AN3008 Application note

AN3008

Application note

STOD2540, single inductor DC-DC converter generates multiple supply voltages for E-paper display

Introduction

This application note describes how to use the STOD2540 DC-DC converter to generate two output voltages using a single inductor and an external charge pump. The circuit shown in Figure 1 generates a 70 V output from a 3.7 V input voltage.

The STOD2540 is a highly integrated boost converter that can provide an adjustable output up to 35 V from a 3.0 to 5.5 V input voltage.

The STOD2540 operates in PFM (pulsed frequency modulation) mode. PFM control simply means that the part only switches when the charge needs to be delivered to the output in order to keep the output voltage regulated.

The converter is ideal for generating the necessary voltages to supply thin-film transistor (TFT) LCDs, OLEDs and E-paper shelf labels. The low operating supply current makes the device ideal for small, portable, battery supplied applications. In shutdown mode the load is disconnected from the input and the quiescent current is less than 3 µA.

Figure 1. High voltage power supply based on STOD2540

	L1			C1
	U1
1			8	D1
	VIN	SW
7	ENABLE	Vcap	6		C2
CIN
			5
2	RSET	Vo
3	AGND	FB	4	D2	D3
	9	PGND
STOD2540					R3	COUT
					550k
CIN: 4.7μF					R4
					10k
COUT: 2 x 1 μF 100 V
C1: 100 nF 50 V
C2: 4.7 μF 50 V
L1: 4.7 μH
D1, D2, D3: STPS2L40AF

January 2010

Doc ID 16021 Rev 2

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www.st.com

Contents	AN3008

Contents

1	High voltage power supply based on STOD2540 . . . . . . . . . . . . . . . . . .		3
	1.1	STOD2540 function description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.2	Load disconnect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.3	Output adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.4	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.5	COUT selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.6	Diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.7	Single inductor circuit based on STOD2540 derives 35 V / 70 V . . . . . . . .	5

2	Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	2.1	Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.2	Output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.3	Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.4	Line regulation 70 V / 35 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.5	Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9

3	Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	3.1	Input / output connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
4	Application schematic and bill of materials . . . . . . . . . . . . . . . . . . . . .		12
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13

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AN3008	High voltage power supply based on STOD2540

1 High voltage power supply based on STOD2540

1.1STOD2540 function description

The STOD2540 uses a PFM control scheme to reach high efficiency in low load conditions. The DC-DC has a current mode control scheme that uses a minimum OFF time and a maximum ON time.

The converter monitors the output voltage through the resistor dividers R1 and R2 by comparing the feedback voltage with the internal reference voltage of 1.24 V.

The integrated main power switch is turned on as soon as the feedback voltage falls below the internal reference. The switch stays on until the inductor current reaches the peak current limit or for a maximum ON time equal to 5.5 µsec. The peak current limit value is adjustable through an external resistor connected between the RSET pin and GND. The main switch stays off for at least a minimum OFF time (300 ns typical) and remains in the off state for as long as the feedback voltage remains above the internal reference voltage.

During the ON time, the load current is only supplied by the charge stored in the output capacitor until the feedback voltage drops below the reference voltage again.

PFM regulation is particularly useful when output currents are low and the part is prevalently in the OFF state.

1.2Load disconnect

When the device is in shutdown mode, a DC current path exists between the power source and the load. A high-side switch LDS isolates the load from the source when the device is disabled.

1.3Output adjust

Choose the R4 value in the range of 10 to 200 kΩ. The value of R3 can be calculated from the following equation.

Equation 1

						VOUT
	R	U	= R	L	×		− 1
						VFB

Where

RU is the upper resistor of the voltage divider.

RL is the lower resistor of the voltage divider.

1% tolerance resistors should be chosen for a more accurate VOUT.

The STOD2540 shows a pulses burst behavior that causes a high output voltage ripple. To decrease the output ripple it is possible to insert a capacitor across the upper feedback resistor. The following formula can be used to obtain a first estimation of the value of the capacitor.

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High voltage power supply based on STOD2540	AN3008

Equation 2

	CF =	1
		2 × π ×	FSW	× RU
		20

Where

RU is the upper resistor of the voltage divider.

FSW is the switching frequency.

The following equation gives the switching frequency at the nominal load current.

Equation 3

FSW (ILOAD	) =	2 × ILOAD	× (VOUT − VIN )
			2
		L × IPK

The CF capacitor increases the amplitude of the voltage ripple on the FB pin, causing a deterioration of the line regulation; therefore, the value of CF should be as small as possible.

1.4Inductor selection

Since the hysteretic control scheme is inherently stable, the inductor value does not affect the stability of the regulator. Using the PFM peak current control scheme, the converter operates in discontinuous conduction mode (DCM).

The inductance value must be calculated so as to ensure that the inductor current reaches the current limit before the maximum ON time expires. The following equation can be used to calculate the maximum value of the inductance.

Equation 4

L ≤ VIN _ MIN × TON _ MAX

IPK

Where IPK is the controlled inductor peak current.

In this case the maximum value of the load current is given by Equation 5.

Equation 5

			I	2	× L
ILOAD _ MAX	=		PK
							× L
					IPK
		2 × (VOUT	+ Vd − VIN ) ×					+ toffMIN
					VIN

1.5COUT selection

The output voltage ripple very much depends on the application conditions. The output capacitor has a significant effect on the output voltage ripple magnitude because it supplies the load current through the charge stored during the ON state.

The output voltage ripple consists of two parts: the first is caused by the ESR, the second by the charging and discharging process of the output capacitor.

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The output ripple can be approximately given by the following equation.

Equation 6

	I	OUT		1		I × L
						PK
VOUT =			×		−
	COUT
			FSW			VOUT + VD − VIN

The magnitude of the ripple will typically be linearly proportional to the output capacitance present. For the best output voltage filtering, a low ESR output capacitor is recommended.

1.6Diode selection

The output diode in a boost converter conducts current only when the power switch is off. The average current is equal to the output current and the maximum current is equal to the peak inductor current.

To maximize efficiency, we recommend using a Schottky diode characterized by:

1.a small forward voltage drop.

2.a rated current larger than the peak inductor current.

3.a reverse voltage larger than the output voltage.

4.a small reverse leakage current.

1.7Single inductor circuit based on STOD2540 derives 35 V/70 V

The circuit shown in Figure 2 is capable of deriving +35 / +70 V from a [3; 5.5] input voltage range. The STOD2540 DC-DC converter generates the 35 V output voltage. The addition of an external charge pump consisting of two Schottky diodes (D2 and D3) and two capacitors (C1 and C2) allows delivering output voltages of over 70 V.

In steady-state operation, the voltage on C2 is 35 V and the voltage on COUT is 70 V. During the ON time the main switch is closed and the current flows from the input to ground through

L1 and the internal switch. During this time, the voltage at node SW is 0 V and C1 is charged up to 35 V. In these conditions, D1 is reverse-biased, D2 is forward-biased, D3 is reversebiased and the load current is supplied only by the output capacitor COUT.

Figure 2. External charge pump - TON state

	L1	SW = 0 V	C1
			+
	CIN
			35 V
			D1
			C2

35 V

70 V

											COUT
D2	D3

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