ST AN938 Application note

AN938

Application note

Designing with L4973, 3.5 A high efficiency

DC-DC converter

Introduction

The L4973 family is a 3.5 A monolithic DC-DC converter in step-down topology operating in continuous mode. It uses BCD60 II technology and is available in two plastic packages, power DIP18 (12+3+3) and SO20 (12+4+4).

Two versions of the device are available. The L4973V3.3 sets the output voltage without any voltage divider at 3.3 V and the L4973V5.1 at 5.1 V.

Both the regulators can control higher output voltage values by using an external voltage divider.

The operating input supply voltage ranges from 8 V to 55 V, while the absolute value with no load is 60 V.

New internal design solutions and superior technological performance have allowed us to develop and produce a device with improved efficiency in all operating conditions and with fewer external components.

While internal current limiting and thermal shutdown are today considered standard protection functions mandatory for a safe load supply, an oscillator with voltage feed-forward improves line regulation and overall control loop. The soft-start function does not allow output overvoltages at turn-on, and the synchronization function can reduce EMI problems in multi-output power supplies. The inhibit function, introduced for power management in equipment having standby features, when active (high), reduces device power consumption by a few tens of µA.

A demonstration board for the L4973 is available through order code EVAL4973 (see

Figure 1).

Figure 1. EVAL4973 demonstration board

AM03483v1

April 2009

Doc ID 5410 Rev 10

1/33

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Contents	AN938

Contents

1	Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	Power supply, UVLO and voltage reference . . . . . . . . . . . . . . . . . . . . . .		6
	2.1	Oscillator, sync and voltage feed-forward . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.2	Current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	2.3	Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
	2.4	Feedback disconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	2.5	Zero load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	2.6	Output overvoltage protection (OVP) . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	2.7	Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	2.8	Turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	2.9	Turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16

2.9.1 Synchronization function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.9.2 Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3	Typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
	3.1	Electrical specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	3.2	Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	3.3	Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	3.4	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
	3.5	Output capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
	3.6	Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
	3.7	Error amplifier and compensation blocks . . . . . . . . . . . . . . . . . . . . . . . . .	22
	3.8	LC filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
	3.9	PWM gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	23

4	Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		27
	4.1	Mains transformer power supply with output adjustable from 0 V to 24 V	27
	4.2	Higher input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
	4.3	Buck-boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
	4.4	Current generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30

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AN938			Contents
	4.5	From positive input to negative output . . . . . . . . . . . . . . . . . . . . . .	. . . . . 30
	4.6	Negative boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 31
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 32

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List of figures	AN938

List of figures

Figure 1.	L4973 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 1
Figure 2.	Pin out - power DIP18 and SO20 packages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 5
Figure 3.	Device block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 5
Figure 4.	Turn-on sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
Figure 5.	Oscillator internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
Figure 6.	Device switching frequency vs. Rosc and Cosc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 7
Figure 7.	Voltage feed-forward function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 8
Figure 8.	Maximum duty cycle vs. Rosc and Cosc as parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 8
Figure 9.	Vo-Io output characteristics - hiccup protection limits output power . . . . . . . . . . . . . . . . . .	. 9
Figure 10.	Vo-Io output characteristics - effective pulse-by-pulse protection. . . . . . . . . . . . . . . . . . . .	. 9
Figure 11.	Schematic of internal current limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 12.	Output current and soft-start voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 13.	Maximum soft-start capacitance with fSW = 100 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 14.	Maximum soft-start capacitance with fSW = 200 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 15.	Soft-start internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
Figure 16.	Soft-start time vs. Vo and Css . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
Figure 17.	Output rising voltage with Css 680 nF, 470 nF, 330 nF, 220 nF . . . . . . . . . . . . . . . . . . . .	13
Figure 18.	Turn-on and turn-off (pin 2, 3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 19.	Power stage internal circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 20.	Sync and oscillator waveforms as slave and as master . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 21.	Re-start output voltage when inhibit goes low (Css = 56 nF) . . . . . . . . . . . . . . . . . . . . . . .	16
Figure 22.	Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
Figure 23.	Input capacitance RMS current vs. duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
Figure 24.	Example of output regulated voltage lower than 3.3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 25.	Output voltage vs. R5 using R3 as parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 26.	Ideal inductor value requested for 30% ripple current, as a function of max. input voltage
	and output (fSW=150 kHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 27.	Ideal inductor value requested for 30% ripple current, as a function of max. input voltage
	and output (fSW=200 kHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 28.	Ideal ripple voltage as a function of input and output voltage (fSW=150 kHz). . . . . . . . . . .	20
Figure 29.	Output drop (%) vs minimum input voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
Figure 30.	Block diagram compensation loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 31.	LC output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 32.	Error amplifier equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 33.	Gain bode plot, open loop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
Figure 34.	Phase bode plot, open loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
Figure 35.	Demonstration board (component side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
Figure 36.	Demonstration board (solder side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
Figure 37.	Typical adjustable 0-24 V power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
Figure 38.	Output voltage vs. R4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
Figure 39.	Design example for 70 V input supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
Figure 40.	Up/down converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
Figure 41.	Constant current generator up to 3.5 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
Figure 42.	Negative output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
Figure 43.	Negative boost converter.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

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AN938	Description

1 Description

For a better understanding of the device and operation, a short description of the main building blocks is given in Figure 2 and 3 with packaging options and complete block diagram.

Figure 2. Pin out - power DIP18 and SO20 packages

	OSC	1	18	SYNC	OSC	1	20	SYNC
	OUT	2	17	SS	OUT	2	19	SS
	OUT	3	16	V5.1	OUT	3	18	V5.1
	GND	4	15	GND	GND	4	17	GND
					GND	5	16	GND
	GND	5	14	GND
					GND	6	15	GND
	GND	6	13	GND	GND	7	14	GND
	VCC	7	12	VFB	VCC	8	13	VFB
	VCC	8	11	COMP	VCC	9	12	COMP
	BOOT	9	10	INH	BOOT	10	11	INH
			D94IN162A			D94IN163A
		Powerdip (12+3+3)				SO20 (12+4+4)		AM03484v1

Figure 3. Device block diagram

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Power supply, UVLO and voltage reference	AN938

2 Power supply, UVLO and voltage reference

The device is provided with an internal stabilized power supply (of about 12 V typ.) that powers the analog and digital control blocks and the bootstrap section.

Moreover, a safe turn-on sequence is guaranteed by an internal UVLO.

When the supply voltage reaches about 3.8 V, the band gap goes into regulation, while the internal 5.1 V reference starts to increase from zero to its nominal value.

Figure 4. Turn-on sequence

AM03486v1

At 6.5 V of Vcc, the device begins charging the soft-start capacitor and the power stage generates the first PWM pulses.

The output voltage increases with a slope controlled by the soft-start time. Figure 4 shows the turn-on sequence of the signals.

From the 12 V pre-regulator, a stable 5.1 V ±2% reference voltage, externally available, is generated, with 10 mA of current capability.

This reference is available on both devices, while the feedback reference is 5.1 V for the L4973V5.1 and 3.3 V for the L4973V3.3.

Figure 5. Oscillator internal circuit

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6/33	Doc ID 5410 Rev 10

AN938	Power supply, UVLO and voltage reference

2.1Oscillator, sync and voltage feed-forward

One pin implements the oscillator function with inherent voltage feed-forward. A second pin is dedicated to in/out synchronization.

A resistor Rosc and a capacitor Cosc, connected as shown in Figure 5, allow the setting of the desired switching frequency given in Equation 1:

Equation 1

1

FSW = ----------------------------------------------------------------------------------

( ) 6

Rosc Cosc In --5 + 100 Cosc

where Fsw is in kHz, Rosc in kΩ and Cosc in nF.

The oscillator capacitor Cosc is discharged by an internal mos transistor of 100 Ω of RDS(on) (Q1) and during this period the internal threshold is set at 1 V by a second mos, Q2. When

the oscillator voltage capacitor reaches the 1 V threshold, the output comparator turns off mos Q1 and turns on mos Q2, charging the external capacitor Cosc. The oscillator block, shown in Figure 5, generates a sawtooth wave signal that sets the switching frequency of the system.

This signal, compared with the output of the error amplifier, generates the PWM signal that modulates the conduction time of the power output stage.

The design of the oscillator implements the voltage feed-forward function without any additional external components.

Figure 6. Device switching frequency vs. Rosc and Cosc

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Power supply, UVLO and voltage reference	AN938

Figure 7. Voltage feed-forward function.

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The oscillator peak-to-valley voltage is proportional to the supply voltage, and the voltage feed-forward is operative from 8 V to 55 V of input supply.

Equation 2

V VCC – 1 osc = --------------------

6

Also the V/ t of the sawtooth is directly proportional to the supply voltage. As Vcc increases, the Ton time of the power transistor decreases in such a way to provide to the choke, and finally also the load, the product volt. x sec. as a constant.

Figure 8 shows how the duty cycle varies as a result of the change on the V/ t of the sawtooth with Vcc. The output of the error amplifier doesn’t change to maintain the output voltage constant and in regulation. With this function on the board, the output response time is greatly reduced in presence of an abrupt change on the supply voltage, and the output ripple voltage at the mains frequency is greatly reduced too.

Figure 8. Maximum duty cycle vs. Rosc and Cosc as parameter

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8/33	Doc ID 5410 Rev 10

AN938	Power supply, UVLO and voltage reference

In fact, the slope of the ramp is modulated by the input ripple voltage, generally present in the order of some tenths of a volt, for both offline and DC-DC converters using mains transformers.

The charge and discharge time is approximately equal to:

Equation 3

Tch	= Rosc Cosc	6
		In --
		5

Tdis = 100 Cosc

The maximum duty cycle is a function of Tch, Tdis and an internal delay and is represented by the equation below:

Equation 4				6	– 80 10	–9
	Rosc Cosc In			--
Dmax	=			5
		6
	Rosc				+ 100 Cosc
		Cosc In	--
			5
which is represented in Figure 9.
Figure 9. Vo-Io output characteristics -	Figure 10. Vo-Io output characteristics -
hiccup protection limits output				effective pulse-by-pulse protection
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2.2Current protection

The L4973 has two current limiting functions: pulse-by-pulse and hiccup modes.

Increasing the output current to the pulse-by-pulse current limiting threshold (Ith1 typ. value of 4.5 A), the controller reduces the on-time to the value of TB = 300 ns (the blanking time at which the current limiting protection does not trigger). This minimum time is necessary to avoid undesirable intervention of the protection due to the spike current generated during the recovery time of the freewheeling diode.

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Power supply, UVLO and voltage reference	AN938

In this condition, because of the fixed blanking time, the output current is given by:

Equation 5

Imax = [R

[VCC TB FSW – Vf (1 – TB FSW)]

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where Ro is the load resistance and Vf is the diode forward voltage. RD and RL are the series resistance of, respectively, the freewheeling diode and the choke.

Typical output characteristics are given in Figure 9 and 10. In Figure 10, the pulse-by-pulse protection is effective in limiting the output current.

In Figure 9 the pulse-by-pulse protection is no longer effective in limiting the current due to the minimum Ton fixed by the blanking time TB, and the hiccup protection intervenes because the output peak current reaches the relative threshold.

Figure 11. Schematic of internal current limiting

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Figure 12. Output current and soft-start voltage

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