ST L6911D User Manual

L6911D

L6911D

5 BIT PROGRAMMABLE STEP DOWN CONTROLLER WITH SYNCHRONOUS RECTIFICATION

■OPERATING SUPPLY IC VOLTAGE FROM 5V TO 12V BUSES

■UP TO 1.3A GATE CURRENT CAPABILITY

■TTL-COMPATIBLE 5 BIT PROGRAMMABLE OUTPUT COMPLIANT WITH VRM 9.0 : 1.100V TO 1.850V WITH 0.025V BINARY STEPS

■VOLTAGE MODE PWM CONTROL

■EXCELLENT OUTPUT ACCURACY: ±1% OVER LINE AND TEMPERATURE VARIATIONS

■VERY FAST LOAD TRANSIENT RESPONSE: FROM 0% TO 100% DUTY CYCLE

■POWER GOOD OUTPUT VOLTAGE

■OVERVOLTAGE PROTECTION AND MONITOR

■OVERCURRENT PROTECTION REALIZED USING THE UPPER MOSFET'S RdsON

■200KHz INTERNAL OSCILLATOR

■OSCILLATOR EXTERNALLY ADJUSTABLE FROM 50KHz TO 1MHz

■SOFT START AND INHIBIT FUNCTIONS

APPLICATIONS

■POWER SUPPLY FOR ADVANCED MICROPROCESSOR CORE

■DISTRIBUTED POWER SUPPLY

■HIGH POWER DC-DC REGULATORS

BLOCK DIAGRAM

SO-20

ORDERING NUMBERS: L6911D

L6911DTR (Tape and Reel)

DESCRIPTION

The device is a power supply controller specifically designed to provide a high performance DC/DC conversion for high current microprocessors. A precise 5-bit digital to analog converter (DAC) allows adjusting the output voltage from 1.30V to 2.05V with 50mV binary steps and from 2.10V to 3.50V with 100mV binary steps.

The high precision internal reference assures the selected output voltage to be within ±1%. The high peak current gate drive affords to have fast switching to the external power mos providing low switching losses.

The device assures a fast protection against load overcurrent and load overvoltage. An external SCR is triggered to crowbar the input supply in case of hard over-voltage. An internal crowbar is also provided turning on the low side mosfet as long as the overvoltage is detected. In case of over-current detection, the soft start capacitor is discharged and the system works in HICCUP mode.

	Vcc 5 to 12V
				Vin 5V to12V
PGOOD	VCC	OCSET
SS			BOOT
	MONITOR and		UGATE
	PROTECTION
OVP
				Vo
RT	OSC		PHASE	1.100V to 1.850V
VD0			LGATE
VD1			PGND
VD2	D/A	-	GND
VD3		+
VD4	+	PWM	VSEN
	-		VFB
	E/A
D98IN957_2	COMP

November 2001

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L6911D

ABSOLUTE MAXIMUM RATINGS

Symbol	Parameter	Value	Unit
VCC	VCC to GND, PGND	15	V
VBOOT-VPHASE	Boot Voltage	15	V
VHGATE-VPHASE		15	V
	OCSET, LGATE, PHASE	-0.3 to Vcc+0.3	V
	RT, SS, FB, PGOOD, VSEN, VID0-4	7	V
	OVP, COMP	6.5	V

THERMAL DATA

Symbol	Parameter	Value	Unit
Rth j-amb	Thermal Resistance Junction to Ambient	110	°C/W
Tj	Maximum junction temperature	150	°C
Tstg	Storage temperature range	-40 to 150	°C
TJ	Junction temperature range	0 to 125	°C

PIN CONNECTION (Top view)

VSEN	1	20	RT
OCSET	2	19	OVP
SS/INH	3	18	VCC
VID0	4	17	LGATE
VID1	5	16	PGND
VID2	6	15	BOOT
VID3	7	14	UGATE
VID4	8	13	PHASE
COMP	9	12	PGOOD
FB	10	11	GND
		D98IN958

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			L6911D
	PIN FUNCTION
	Pin	Name	Description
	Num.
	1	VSEN	Connected to the output voltage is able to manage over-voltage conditions and the PGOOD
			signal.
	2	OCSET	A resistor connected from this pin and the upper Mos Drain sets the current limit protection.
			The internal 200mA current generator sinks a current from the drain through the external resistor.
			The Over-Current threshold is due to the following equation:
			IOCSE T × RO CS ET
			IP = ---------------------------------------------
			RDS on
	3	SS/INH	The soft start time is programmed connecting an external capacitor from this pin and GND. The
			internal current generator forces through the capacitor 10mA.
			This pin can be used to disable the device forcing a voltage lower than 0.4V
	4 - 8	VID0 - 4	Voltage Identification Code pins. These input are internally pulled-up and TTL compatible. They
			are used to program the output voltage as specified in Table 1 and to set the overvoltage and
			power good thresholds.
			Connect to GND to program a ‘0’ while leave floating to program a ‘1’.
	9	COMP	This pin is connected to the error amplifier output and is used to compensate the voltage control
			feedback loop.
	10	FB	This pin is connected to the error amplifier inverting input and is used to compensate the voltage
			control feedback loop.
	11	GND	All the internal references are referred to this pin. Connect it to the PCB signal ground.
	12	PGOOD	This pin is an open collector output and is pulled low if the output voltage is not within the above
			specified thresholds.
			If not used may be left floating.
	13	PHASE	This pin is connected to the source of the upper mosfet and provides the return path for the high
			side driver. This pin monitors the drop across the upper mosfet for the current limit
	14	UGATE	High side gate driver output.
	15	BOOT	Bootstrap capacitor pin. Through this pin is supplied the high side driver and the upper mosfet.
			Connect through a capacitor to the PHASE pin and through a diode to Vcc (cathode vs. boot).
	16	PGND	Power ground pin. This pin has to be connected closely to the low side mosfet source in order to
			reduce the noise injection into the device
	17	LGATE	This pin is the lower mosfet gate driver output
	18	VCC	Device supply voltage. The operative nominal supply voltage ranges from 5 to 12V.
			DO NOT CONNECT VIN TO A VOLTAGE GREATER THAN VCC.
	19	OVP	Over voltage protection. If the output voltage reaches the 17% above the programmed voltage
			this pin is driven high and can be used to drive an external SCR that crowbar the supply voltage.
			If not used, it may be left floating.
	20	RT	Oscillator switching frequency pin. Connecting an external resistor from this pin to GND, the
			external frequency is increased according to the equation:
			4.94 × 106
			fS = 200kHz + -------------------------
			RT(kW)
			Connecting a resistor from this pin to Vcc (12V), the switching frequency is reduced according to
			the equation:
			4.306 × 107
			fS = 200kHz – ----------------------------
			RT (kW )
			If the pin is not connected, the switching frequency is 200KHz.
			The voltage at this pin is fixed at 1.23V (typ). Forcing a 50mA current into this pin, the built in
			oscillator stops to switch.
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L6911D

ELECTRICAL CHARACTERISTCS (VCC = 12V, Tamb = 25°C unless otherwise specified)

Symbol	Parameter	Test Condition	Min.	Typ.	Max.	Unit
VCC SUPPLY CURRENT
Icc	Vcc Supply current	UGATE and LGATE open		5		mA
POWER-	ON
	Turn-On Vcc threshold	VOCSET=4.5V			4.6	V
	Turn-Off Vcc threshold	VOCSET=4.5V	3.6			V
	Rising VOCSET threshold			1.24		V
ISS	Soft start Current			10		μA
OSCILLATOR
	Free running frequency	RT = OPEN	180	200	220	KHz
	Total Variation	6 KΩ < RT to GND < 200 KΩ	-15		15	%
Vosc	Ramp amplitude	RT = OPEN		1.9		Vp-p
REFERENCE AND DAC
	DACOUT Voltage	VID0, VID1, VID2, VID3, VID4	-1		1	%
	Accuracy	see Table1; Tamb = 0 to 70°C
	VID Pull-Up voltage			4		V
ERROR	AMPLIFIER
	DC Gain			88		dB
GBWP	Gain-Bandwidth Product			10		MHz
SR	Slew-Rate	COMP=10pF		10		V/μS
GATE DRIVERS
IUGATE	High Side Source	VBOOT - VPHASE=12V,	1	1.3		A
	Current	VUGATE - VPHASE= 6V
RUGATE	High Side Sink	VBOOT-VPHASE=12V,		2	4	Ω
	Resistance	IUGATE = 300mA
ILGATE	Low Side Source	Vcc=12V, VLGATE = 6V	0.9	1.1		A
	Current
RLGATE	Low Side Sink	Vcc=12V, ILGATE = 300mA		1.5	3	Ω
	Resistance
	Output Driver Dead Time	PHASE connected to GND		120		ns
PROTECTIONS
	Over Voltage Trip	VSEN Rising		117	120	%
	(VSEN/DACOUT)
IOCSET	OCSET Current Source	VOCSET = 4.5V	170	200	230	μA
IOVP	OVP Sourcing Current	VSEN > OVP Trip, VOVP=0V	60			mA
POWER	GOOD
	Upper Threshold	VSEN Rising	110	112	114	%
	(VSEN/DACOUT)
	Lower Threshold	VSEN Falling	86	88	90	%
	(VSEN/DACOUT)
	Hysteresis	Upper and Lower threshold		2		%
	(VSEN/DACOUT)
VPGOOD	PGOOD Voltage Low	IPGOOD = -5mA		0.5		V

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												L6911D
Table 1. VID Settings
VID4	VID3	VID2		VID1	VID0	Output	VID4	VID3	VID2	VID1	VID0	Output Voltage
						Voltage (V)						(V)
1	1	1		1	1	Output OFF	0	1	1	1	1	1.475
1	1	1		1	0	1.100	0	1	1	1	0	1.500
1	1	1		0	1	1.125	0	1	1	0	1	1.525
1	1	1		0	0	1.150	0	1	1	0	0	1.550
1	1	0		1	1	1.175	0	1	0	1	1	1.575
1	1	0		1	0	1.200	0	1	0	1	0	1.600
1	1	0		0	1	1.225	0	1	0	0	1	1.625
1	1	0		0	0	1.250	0	1	0	0	0	1.650
1	0	1		1	1	1.275	0	0	1	1	1	1.675
1	0	1		1	0	1.300	0	0	1	1	0	1.700
1	0	1		0	1	1.325	0	0	1	0	1	1.725
1	0	1		0	0	1.350	0	0	1	0	0	1.750
1	0	0		1	1	1.375	0	0	0	1	1	1.775
1	0	0		1	0	1.400	0	0	0	1	0	1.800
1	0	0		0	1	1.425	0	0	0	0	1	1.825
1	0	0		0	0	1.450	0	0	0	0	0	1.850

Device Description

The device is an integrated circuit realized in BCD technology. It provides complete control logic and protections for a high performance step-down DC-DC converter optimized for microprocessor power supply. It is designed to drive N-Channel Mosfets in a synchronous-rectified buck topology. The device works properly with Vcc ranging from 5V to 12V and regulates the output voltage starting from a 1.26V power stage supply voltage (Vin). The output voltage of the converter can be precisely regulated, programming the VID pins, from 1.100V to 1.850V with 25mV binary steps, with a maximum tolerance of ±1% over temperature and line voltage variations. The device provides voltage-mode control with fast transient response. It includes a 200kHz free-running oscillator that is adjustable from 50kHz to 1MHz.

The error amplifier features a 15MHz gain-bandwidth product and 10V/ms slew rate which permits high converter bandwidth for fast transient performance. The resulting PWM duty cycle ranges from 0% to 100%. The device protects against over-current conditions entering in HICCUP mode. The device monitors the current by using the rDS(ON) of the upper MOSFET which eliminates the need for a current sensing resistor.

The device is available in SO20 package

Oscillator

The switching frequency is internally fixed to 200kHz. The internal oscillator generates the triangular waveform for the PWM charging and discharging with a constant current an internal capacitor. The current delivered to the oscillator is typically 50mA (Fsw=200KHz) and may be varied using an external resistor (RT) connected between RT pin and GND or VCC. Since the RT pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied proportionally to the current sunk (forced) from (into) the pin.

In particular connecting it to GND the frequency is increased (current is sunk from the pin), according to the following relationship:

4.94 × 106 f = 200kHz + -------------------------

S RT (kW)

Connecting RT to VCC=12V or to VCC=5V the frequency is reduced (current is forced into the pin), according to the following relationships:

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L6911D

fS	= 200kHz +	4.306 × 10			7
		-----	R----T----(--k---W------)---		- VCC = 12V
	fS = 200kHz	+	15	× 107	VCC = 5V
			R-----T---	(---k---W-----)-

Switching frequency variations vs. RT are reported in Fig.1.

Note that forcing a 50mA current into this pin, the device stops switching because no current is delivered to the oscillator.

Figure 1.

	1 0 00 0
	1 00 0
ce [kO h m ]	10 0
R e s is ta n		RT to G N D
	1 0
		RT to V CC =12V
		RT to V CC =5V
	1 0	1 0 0	1 0 0 0

Fre quency [kH z]

Digital to Analog Converter

The built-in digital to analog converter allows the adjustment of the output voltage from 1.30V to 2.05V with 50mV binary steps and from 2.10V to 3.50V with 100mV binary steps as shown in the previous table 1. The internal reference is trimmed to ensure the precision of 1%.

The internal reference voltage for the regulation is programmed by the voltage identification (VID) pins. These are TTL compatible inputs of an internal DAC that is realized by means of a series of resistors providing a partition of the internal voltage reference. The VID code drives a multiplexer that selects a voltage on a precise point of the divider. The DAC output is delivered to an amplifier obtaining the VPROG voltage reference (i.e. the set-point of the error amplifier). Internal pull-ups are provided (realized with a 5mA current generator); in this way, to program a logic "1" it is enough to leave the pin floating, while to program a logic "0" it is enough to short the pin to GND.

The voltage identification (VID) pin configuration also sets the power-good thresholds (PGOOD) and the overvoltage protection (OVP) thresholds.

The VID code "11111" disable the device (as a short on the SS pin) and no output voltage is regulated.

Soft Start and Inhibit

At start-up a ramp is generated charging the external capacitor CSS by means of a 10µA constant current, as shown in figure 1.

When the voltage across the soft start capacitor (VSS) reaches 0.5V the lower power MOS is turned on to dis-

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