ST L6728AH User Manual

High frequency single phase PWM controller with Power Good

Features

■ Flexible power supply from 5 V to 12 V

■ Power conversion input as low as 1.5 V

■ 0.8 V internal reference

■ 0.8% output voltage accuracy

■ High-current integrated drivers

■ Power Good output

■ Sensorless and programmable OCP across

low-side R

■

OV / UV protections

■ VSEN disconnection protection

■ Oscillator internally fixed at 600 kHz

■ LS-LESS to manage pre-bias start-up

■ Adjustable output voltage

■ Disable function

■ Internal soft-start

■ VFDFPN 10 package

Applications

DS(on)

L6728AH

VFQFPN 10

Description

L6728AH is a single-phase step-down controller
with integrated high-current drivers that provides
complete control logic and protection to realize in
a simple way general DC-DC converters by using
a compact VFDFPN 10 package.

Device flexibility allows managing conversions
with power input V
supply voltage ranging from 5 V to 12 V.

L6728AH provides simple control loop with
voltage mode EA. The integrated 0.8 V reference
allows regulating output voltages with ±0.8%
accuracy over line and temperature variations.
Oscillator is internally fixed to 600 kHz.

as low as 1.5 V and device

IN

■ Memory and termination supply

■ Subsystem power supply (MCH, IOCH, PCI)

■ CPU and DSP power supply

■ Distributed power supply

■ General DC / DC converters

L6728AH provides programmable dual level over
current protection as well as over and under
voltage protection. Current information is
monitored across the low-side MOSFET R
saving the use of expensive and space­consuming sense resistors.

PGOOD output easily provides real-time
information on output voltage status, through
VSEN dedicated output monitor.

Table 1. Device summary

Order codes Package Packing

L6728AH

Tube

VFDFPN 10

L6728AHTR Tape and reel

May 2009 Doc ID 15726 Rev 1 1/33

DS(on)

www.st.com

33

Contents L6728AH

Contents

1 Typical application circuit and block diagram . . . . . . . . . . . . . . . . . . . . 4

1.1 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Pins description and connection diagrams . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Pin descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.1 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.1 Low-side-less start up (LSLess) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7 Over-current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.1 Over-current threshold setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8 Output voltage setting and protections . . . . . . . . . . . . . . . . . . . . . . . . 13

9 Application details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

9.1 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

9.2 Layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

10 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

10.1 Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

10.2 Output capacitor(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

10.3 Input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2/33 Doc ID 15726 Rev 1

L6728AH Contents

11 20 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11.1 Demonstration board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.1.1 Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.1.2 Output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.1.3 Signal input (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.1.4 Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11.2 Demonstration board characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

12 5 A demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

12.1 Demonstration board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.1.1 Power input (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.1.2 Output (VOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.1.3 Signal input (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.1.4 Test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12.2 Demonstration board characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

13 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

14 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Doc ID 15726 Rev 1 3/33

Typical application circuit and block diagram L6728AH

1 Typical application circuit and block diagram

1.1 Application circuit

Figure 1. Typical application circuit

C

HF

VIN = 1.5V to 12V

L

C

BULK

Vout

C

OUT

LOAD

VCC = 5V to 12V

PGOOD

C

P

R

OS

C

DEC

R

PG

10

PGOOD

7

COMP

C

F

R

F

/ DIS

8

FB

VSEN

R

9

FB

6

VCC

UGATE

PHASE

L6728A

L6728AH

GND

5

BOOT

LGATE

/ OC

R

1

3

2

4

OCSET

HS

LS

L6728A Reference Schematic

1.2 Block diagram

Figure 2. Block diagram

VSEN

PGOOD

R

OS

V

MONITOR

OUT

600 kHz

OSCILLATOR

L6728AH

L6728A

CLOCK

R

FB

VCC

CONTROL LOGIC

PROTECTIONS

ERROR AMPLIFIER

V

OC

OCTH

&

BOOT

CROSS CONDUCTION

ADAPTIVE ANTI

HS

UGATE

PHASE

PWM

VCC

LS

LGATE
/ OC

GND

+

0.8V

­I

OCSET

/ DIS

COMP

4/33 Doc ID 15726 Rev 1

FB

L6728AH Pins description and connection diagrams

2 Pins description and connection diagrams

Figure 3. Pins connection (top view)

BOOT
PHASE
UGATE

LGATE / OC

GND

1

2

3

4

5

L6728A

L6728AH

2.1 Pin descriptions

Table 2. Pin description

Pin # Name Function

1BOOT

2 PHASE

3 UGATE HS driver output. Connect directly to HS MOSFET gate.

4 LGATE / OC

5GND

6VCC

7 COMP / DIS

HS driver supply.
Connect through a capacitor (100 nF) to the floating node (LS-Drain) pin and provide

necessary bootstrap diode from V

CC

.

HS driver return path, current-reading and adaptive-dead-time monitor. Connect to the LS
drain to sense R

drop to measure the output current. This pin is also used by the

DS(on)

adaptive-dead-time control circuitry to monitor when HS MOSFET is OFF.

LGATE. LS driver output. Connect directly to LS MOSFET gate.
OC over-current threshold set. During a short period of time following V

threshold, a 10 μA current is sourced from this pin. Connect to GND with an R
greater than 5 kΩ to program OC Threshold. The resulting voltage at this pin is sampled and
held internally as the OC set point. Maximum programmable OC threshold is 0.55 V. A voltage
greater than 0.6 V activates an internal clamp and causes OC threshold to be set at the
maximum value.

All internal references, logic and drivers are connected to this pin.
Connect to the PCB ground plane.

Device and drivers power supply.
Operative range from 5 V to 12 V. Filter with at least 1 μF MLCC to GND.

COMP. Error amplifier output. Connect with an R
control loop.

DIS. The device can be disabled by pushing this pin lower than 0.75 V (typ). Setting free the
pin, the device enables again.

10

PGOOD

9

VSEN

8

FB

7

COMP / DIS

6

VCC

rising over UVLO

CC

- CF // CP to FB to compensate the device

F

OCSET

resistor

Error amplifier inverting input.

8FB

Connect with a resistor R

to the output regulated voltage. Output resistor divider may be

FB

used to regulate voltages higher than the reference.

9 VSEN

Regulated voltage sense pin for OVP and UVP protections and PGOOD. Connect to the
output regulated voltage, or to the output resistor divider if the regulated voltage is higher than
the reference.

10 PGOOD

Open drain output set free after SS has finished and pulled low when VSEN is outside the
relative window. Pull up to a voltage equal or lower than V

. If not used it can be left floating.

CC

Doc ID 15726 Rev 1 5/33

Pins description and connection diagrams L6728AH

2.2 Thermal data

Table 3. Thermal data

Symbol Parameter Value Unit

R

TH(JA)

R

TH(JC)

T

T

P

MAX

STG

T

J

TOT

Thermal resistance junction to ambient
(Device soldered on 2s2p, 67 mm x 69 mm board)

45 °C/W

Thermal resistance junction to case 5 °C/W

Maximum junction temperature 150 °C

Storage temperature range -40 to 150 °C

Junction temperature range -40 to 125 °C

Maximum power dissipation at TA = 25 °C 2.25 W

6/33 Doc ID 15726 Rev 1

L6728AH Electrical specifications

3 Electrical specifications

3.1 Absolute maximum ratings

Table 4. Absolute maximum ratings

Symbol Parameter Value Unit

V

CC

V

BOOT, VUGATE

V

PHASE

V

LGATE

to GND -0.3 to 15 V

to PHASE
to GND
to GND; t < 200 ns

to GND
to GND; t < 200 ns

to GND

-5 to 18

-8 to 30

-0.3 to V

15
33
45

CC

+0.3

FB, COMP, VSEN to GND -0.3 to 3.6 V

PGOOD to GND

-0.3 to V

CC

+0.3

3.2 Electrical characteristics

VCC = 5 V to 12 V; TJ = 0 °C to 70 °C unless otherwise specified

Table 5. Electrical characteristics

Symbol Parameter Test conditions Min. Typ. Max. Unit

Supply current and power-on

I

CC

I

BOOT

UVLO

VCC supply current UGATE and LGATE = OPEN 6 mA

BOOT supply current UGATE = OPEN; PHASE to GND 0.7 mA

V

turn-ON VCC rising 4.1 V

CC

Hysteresis 0.2 V

V

V

V

V

Oscillator

F

ΔV

d

SW

OSC

MAX

Main oscillator accuracy 540 600 660 kHz

PWM ramp amplitude 1.4 V

Maximum duty cycle 67 %

Reference and error amplifier

Output voltage accuracy -0.8 - 0.8 %

A

0

GBWP Gain-bandwidth product

DC gain

SR Slew-rate

(1)

(1)

(1)

120 dB

15 MHz

8V/μs

DIS Disable threshold COMP falling 0.70 0.85 V

Doc ID 15726 Rev 1 7/33

Electrical specifications L6728AH

Table 5. Electrical characteristics (continued)

Symbol Parameter Test conditions Min. Typ. Max. Unit

Gate drivers

I

UGATE

R

UGATE

I

LGATE

R

LGATE

HS source current BOOT - PHASE = 5 V 1.5 A

HS sink resistance BOOT - PHASE = 5 V 1.1 Ω

LS source current VCC = 5 V 1.5 A

LS sink resistance VCC = 5 V 0.65 Ω

Over-current protection

I

OCSET

V

OC_SW

OCSET current source

OC switch-over threshold V

Sourced from LGATE pin, during OC
setting phase

LGATE/OC

rising 600 mV

91011μA

Over and under-voltage protections

VSEN rising 0.90 1.00 1.10 V

OVP OVP threshold

unlatch, VSEN falling 0.35 0.40 0.45 V

UVP UVP threshold VSEN falling 0.50 0.60 0.70 V

VSEN VSEN bias current Sourced from VSEN 100 nA

PGOOD

Upper threshold VSEN rising 0.860 0.890 0.920 V

PGOOD

Lower threshold VSEN falling 0.680 0.710 0.740 V

V

PGOODL

1. Guaranteed by design, not subject to test.

PGOOD voltage low I

= -4 mA 0.4 V

PGOOD

8/33 Doc ID 15726 Rev 1

L6728AH Device description

4 Device description

L6728AH is a single-phase PWM controller with embedded high-current drivers that
provides complete control logic and protections to realize in an easy and simple way a
general DC-DC step-down converter. Designed to drive N-channel MOSFETs in a
synchronous buck topology, with its high level of integration this 10-pin device allows
reducing cost and size of the power supply solution also providing real-time PGOOD in a
compact VFQFPN10 3x3 mm.

L6728AH is designed to operate from a 5 V or 12 V supply. The output voltage can be
precisely regulated to as low as 0.8 V with ±1% accuracy over line and temperature
variations. The switching frequency is internally set to 600 kHz.

This device provides a simple control loop with a voltage-mode error-amplifier. The error­amplifier features a 15 MHz gain-bandwidth product and 8 V/µs slew rate, allowing high
regulator bandwidth for fast transient response.

To avoid load damages, L6728AH provides over-current protection as well as overvoltage,
under voltage and feedback disconnection protection. The over-current trip threshold is
programmable by a simple resistor connected from Lgate to GND. Output current is
monitored across low-side MOSFET R
consuming sense resistor. Output voltage is monitored through dedicated VSEN pin.

, saving the use of expensive and space-

DS(on)

L6728AH implements soft-start increasing the internal reference in closed loop regulation.
low-side-less feature allows the device to perform soft-start over pre-biased output avoiding
high current return through the output inductor and dangerous negative spike at the load
side.

L6728AH is available in a compact VFDFN10 3 x 3 mm package with exposed pad.

Doc ID 15726 Rev 1 9/33

Driver section L6728AH

5 Driver section

The integrated high-current drivers allow using different types of power MOSFET (also
multiple MOSFETs to reduce the equivalent R

The driver for the high-side MOSFET uses BOOT pin for supply and PHASE pin for return.
The driver for low-side MOSFET uses the

V

CC

The controller embodies an anti-shoot-through and adaptive dead-time control to minimize
low side body diode conduction time, maintaining good efficiency while saving the use of
Schottky diode:

to check high-side MOSFET turn off, PHASE pin is sensed. When the voltage at PHASE pin
drops down, the low-side MOSFET gate drive is suddenly applied;

to check low-side MOSFET turn off, LGATE pin is sensed. When the voltage at LGATE has
fallen, the high-side MOSFET gate drive is suddenly applied.

If the current flowing in the inductor is negative, voltage on PHASE pin will never drop. To
allow the low-side MOSFET to turn-on even in this case, a watchdog controller is enabled: if
the source of the high-side MOSFET doesn't drop, the low side MOSFET is switched on so
allowing the negative current of the inductor to recirculate. This mechanism allows the
system to regulate even if the current is negative.

), maintaining fast switching transition.

DS(on)

pin for supply and GND pin for return.

Power conversion input is flexible: 5 V, 12 V bus or any bus that allows the conversion (See
maximum duty cycle limitations) can be chosen freely.

5.1 Power dissipation

L6728AH embeds high current MOSFET drivers for both high side and low side MOSFETs:
it is then important to consider the power that the device is going to dissipate in driving them
in order to avoid overcoming the maximum junction operative temperature.

Two main terms contribute in the device power dissipation: bias power and drivers' power.

● Device bias power (P

supply pins and it is simply quantifiable as follow (assuming to supply HS and LS
drivers with the same

● Drivers power is the power needed by the driver to continuously switch on and off the

external MOSFETs; it is a function of the switching frequency and total gate charge of
the selected MOSFETs. It can be quantified considering that the total power P
dissipated to switch the MOSFETs (easy calculable) is dissipated by three main
factors: external gate resistance (when present), intrinsic MOSFET resistance and
intrinsic driver resistance. This last term is the important one to be determined to
calculate the device power dissipation. The total power dissipated to switch the
MOSFETs results:

P

SW

) depends on the static consumption of the device through the

DC

V

of the device):

CC

P

F

DC

SW

V

CCICCIBOOT

Q

gHSVBOOT

+()⋅=

Q

gLSVCC

SW

⋅+⋅()⋅=

External gate resistors helps the device to dissipate the switching power since the same
power P

will be shared between the internal driver impedance and the external resistor

SW

resulting in a general cooling of the device.

10/33 Doc ID 15726 Rev 1