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LTC3828

3828fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Dual, 2-Phase Step-Down

Controller with Tracking

The LTC

3828 is a dual high performance step-down

switching regulator controller that drives all N-channel

synchronous power MOSFET stages. A constant frequency

current mode architecture allows for a phase-lockable

frequency of up to 550kHz. The TRCKSS pin provides both

soft-start and tracking functions. Multiple LTC3828s can

be daisy-chained in applications requiring more than two

voltages to be tracked.

OPTI-LOOP compensation allows the transient response

to be optimized over a wide range of output capacitance

and ESR values. The precision 0.8V reference and power

good output indicator are compatible with a wide 4.5V to

28V (30V maximum) input supply range, encompassing

all battery chemistries.

The RUN pins control their respective channels indepen-

dently. The FCB/PLLIN pin selects among Burst Mode

operation, skip-cycle mode and continuous current mode.

Current foldback limits MOSFET dissipation during short-

circuit conditions. Reverse current and current foldback

functions are disabled during soft-start.

High Efﬁ ciency Dual 2.5V/3.3V Step-Down Converter with Tracking

Telecom Infrastructure

ASIC Power Supply

Industry Equipment

L, LT, LTC, LTM, Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners.

Protected by U.S. Patents including 5481178, 5705919, 5929620, 6144194, 6177787,

6304066, 6580258

Dual, 180° Phased Controllers Reduce Required

Input Capacitance and Power Supply Induced Noise

Tracking for Both Outputs

Constant Frequency Current Mode Control

Wide V

Range: 4.5V to 28V Operation

Power Good Output Voltage Indicator

Adjustable Soft-Start Current Ramping

Foldback Output Current Limiting–

Disabled at Start-Up

No Reverse Current During Soft-Start Interval

Clock Output for 3-, 4-, 6-Phase Operation

Dual N-Channel MOSFET Synchronous Drive

±1% Output Voltage Accuracy

Phase-Lockable Fixed Frequency 260kHz to 550kHz

OPTI-LOOP

Compensation Minimizes C

OUT

Very Low Dropout Operation: 99% Duty Cycle

Output Overvoltage Protection

Small 28-Lead SSOP and 5mm × 5mm QFN

Packages

4.7µF

0.1µF

63.4k 1%

1000pF

3.2µH

220pF

1µF

CERAMIC

22µF

50V

CERAMIC

47µF

0.01

20k

15k

3.3V

0.1µF

42.5k

3.2µH

220pF

1000pF

56µF

0.01

20k

15k

2.5V

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND

PGND

SENSE1

SENSE2

SENSE1

–

SENSE2

–

OSENSE1

OSENSE2

TH1

TH2

PGOOD INTV

RUN1 RUN2

4.5V TO 28V

3828 TA01

LTC3828

FCB/PLLIN

500kHz

TRCKSS1TRCKSS1TRCKSS2

63.4k 1%

Summary of content (32 pages)

PAGE 1
LTC3828 Dual, 2-Phase Step-Down Controller with Tracking DESCRIPTION FEATURES n n n n n n n n n n n n n n n n Dual, 180° Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise Tracking for Both Outputs Constant Frequency Current Mode Control Wide VIN Range: 4.
PAGE 2
LTC3828 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN) ........................ 30V to –0.3V Top Side Driver Voltages BOOST1, BOOST2................................. 36V to –0.3V Switch Voltage (SW1, SW2) ........................ 30V to –5V INTVCC, DRVCC, RUN1, RUN2, (BOOST1-SW1), (BOOST2-SW2)........................................... 7V to –0.3V SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages ........................ (1.1)INTVCC to –0.3V FCB/PLLIN, PLLFLTR, CLKOUT, PHSMD Voltage ................
PAGE 3
LTC3828 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 15V, VRUN1, 2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.792 0.800 0.808 V Main Control Loops VOSENSE1, 2 Regulated Feedback Voltage IVOSENSE1, 2 Feedback Current (Note 3); ITH1, 2 Voltage = 1.2V l ±5 ±50 nA VREFLNREG Reference Voltage Line Regulation (Note 3) VIN = 4.
PAGE 4
LTC3828 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 15V, VRUN1, 2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.1 0.3 V ±1 μA –9.5 9.
PAGE 5
LTC3828 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Supply Current vs Input Voltage (Figure 14) 1200 5.2 BOTH CONTROLLERS ON 3.8 ILOAD = 1mA 800 600 400 200 UNDERVOLTAGE LOCKOUT (V) 5.0 INTVCC VOLTAGE (V) SUPPLY CURRENT (μA) 1000 Undervoltage Lockout vs Temperature Internal 5V LDO Line Regulation 4.8 4.6 4.4 4.2 3.6 3.4 3.2 SHUTDOWN 0 0 5 10 15 20 INPUT VOLTAGE (V) 25 4.0 30 0 5 10 15 20 INPUT VOLTAGE (V) 3828 G04 3.
PAGE 6
LTC3828 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Maximum Current Sense Threshold vs Percent of Nominal Output Voltage (Foldback) Maximum Current Sense Threshold vs Duty Factor 75 Maximum Current Sense Threshold vs Sense Common Mode Voltage 80 80 70 75 25 VSENSE (mV) VSENSE (mV) VSENSE (mV) 60 50 50 40 30 70 65 20 10 0 0 40 20 60 80 60 0 100 0 0.25 0.5 0.75 1.
PAGE 7
LTC3828 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Load Step: Burst Mode Operation (Figure 14) VOUT1 100mV/DIV VOUT1 100mV/DIV VOUT2 100mV/DIV VOUT2 100mV/DIV IL1 2A/DIV IL1 2A/DIV 50μs/DIV VIN = 12V VOUT1 = 5V VOUT2 = 3.3V LOAD STEP = 0A TO 3A WITH RATIOMETRIC TRACKING 3828 G22 SW1 20V/DIV SW2 20V/DIV IIN 25A/DIV VIN RIPPLE 50mV/DIV 50μs/DIV VIN = 12V VOUT1 = 5V VOUT2 = 3.
PAGE 8
LTC3828 PIN FUNCTIONS (SSOP/QFN) SGND (Pin 9/Pin 6): Small-Signal Ground. Common to both controllers, this pin must be routed separately from high current grounds to the common (–) terminals of the COUT capacitors. TRCKSS2, TRCKSS1 (Pins 10, 1/Pins 7, 29): Soft-Start and Output Voltage Tracking Inputs. When one channel is conﬁgured to be the master of two outputs a capacitor to ground at this pin sets the ramp rate.
PAGE 9
LTC3828 BLOCK DIAGRAM PHSMD PHASE LOGIC CLKOUT PHASE DET PLLFLTR CLK1 RLP OSCILLATOR CLK2 CLP – 0.86V + PGOOD VOSENSE1 – + 0.74V – 0.86V 0.8V VREF DRVCC 5V LDO REG + VOSENSE2 – 3V + 0.18μA PLL DETECTOR INTERNAL SUPPLY BINH + SGND (UH PACKAGE PAD) FCB – DRVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL DROP OUT DET S Q R Q TOP – SHDN + + – INTVCC 30k I2 VOUT RSENSE – + CIN COUT PGND SENSE+ – 30k SENSE 45k 45k 1.2μA 2.4V – EA + 0.
PAGE 10
LTC3828 OPERATION (Refer to Functional Diagram) Main Control Loop The IC uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out of phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current comparator, I1, resets the RS latch.
PAGE 11
LTC3828 OPERATION (Refer to Functional Diagram) Frequency Synchronization The phase-locked loop allows the internal oscillator to be synchronized to an external source via the FCB/PLLIN pin. The output of the phase detector at the PLLFLTR pin is also the DC frequency control input of the oscillator that operates over a 260kHz to 550kHz range corresponding to a DC voltage input from 0V to 2.4V. When locked, the PLL aligns the turn on of the top MOSFET to the rising edge of the synchronizing signal.
PAGE 12
LTC3828 OPERATION (Refer to Functional Diagram) THEORY AND BENEFITS OF 2-PHASE OPERATION The LTC3728 and the LTC3828 family of dual high efﬁciency DC/DC controllers brings the considerable beneﬁts of 2-phase operation to portable applications for the ﬁrst time. Notebook computers, PDAs, handheld terminals and automotive electronics will all beneﬁt from the lower input ﬁltering requirement, reduced electromagnetic interference (EMI) and increased efﬁciency associated with 2-phase operation.
PAGE 13
LTC3828 OPERATION (Refer to Functional Diagram) 3.0 SINGLE-PHASE DUAL CONTROLLER INPUT RMS CURRENT (A) 2.5 2.0 1.5 2-PHASE DUAL CONTROLLER 1.0 0.5 0 VO1 = 5V/3A VO2 = 3.3V/3A 0 10 20 30 INPUT VOLTAGE (V) 40 3828 F03 Figure 3. RMS Input Current Comparison APPLICATIONS INFORMATION Output Voltage Tracking The LTC3828 allows the user to program how the channel outputs ramp up by means of the TRCKSS pins.
PAGE 14
LTC3828 APPLICATIONS INFORMATION By selecting different resistors, the LTC3828 can achieve different modes of tracking including the two in Figure 4. Figure 6 helps to explaining the tracking function. At the input stage of an error ampliﬁer, two diodes are used to clamp the equivalent reference voltage and an additional diode is used to match the shifted common mode voltage. The top two current sources are of the same value.
PAGE 15
LTC3828 APPLICATIONS INFORMATION TRCKSS2 VOUT1 R4 R5 R1 R2 R2 R2 LTC3828 “MASTER” VOUT2 R3 VOUT1 TRCKSS1 R2 CSS TRCKSS1 TRCKSS2 VOUT3 R4 LTC3828 “SLAVE” VOUT4 OUTPUT VOLTAGE VOSENSE1 VOSENSE2 VOUT3 VOUT4 R5 VOUT2 VOSENSE1 VOSENSE2 R2 R2 3828 F08 TIME (8a) Circuit Setup (8b) Output Voltage Figure 8. Four Outputs with Tracking and Ratiometric Sequencing R5 VOUT 4 ⎞ ⎛ R1 VOUT1 R3 VOUT2 R4 VOUT3 ⎜⎝ R2 = 0.8 – 1, R2 = 0.8 – 1R2 = 0.88 – 1, R2 = 0.
PAGE 16
LTC3828 APPLICATIONS INFORMATION of MOSFET gate charge losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. The inductor value has a direct effect on ripple current.
PAGE 17
LTC3828 APPLICATIONS INFORMATION Synchronous Switch Duty Cycle = VIN – VOUT VIN The MOSFET power dissipations at maximum output current are given by: VOUT 2 IMAX ) (1 + δ )RDS(ON) + ( VIN ⎞ (VIN )2 ⎛⎜⎝ IMAX ⎟ (RDR )(C MILLER ) • 2 ⎠ PMAIN = ⎡ 1 1 ⎤ + ⎢V ⎥( f ) ⎣ INTVCC – VTHMIN VTHMIN ⎦ PSYNC = ( ) (1+ δ)RDS(ON) VIN – VOUT IMAX VIN 2 where δ is the temperature dependency of RDS(ON) and RDR (approximately 2Ω) is the effective driver resistance at the MOSFET’s Miller threshold voltage.
PAGE 18
LTC3828 APPLICATIONS INFORMATION an ideal candidate for highest efﬁciency battery operated systems. Also consider parallel ceramic and high quality electrolytic capacitors as an effective means of achieving ESR and bulk capacitance goals. In continuous mode, the source current of the top N-channel MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current of one channel must be used.
PAGE 19
LTC3828 APPLICATIONS INFORMATION bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between channels. Higher input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the IC to be exceeded. The system supply current is normally dominated by the gate charge current.
PAGE 20
LTC3828 APPLICATIONS INFORMATION up. The LTC3828 has the ability to either soft-start by itself with an external soft-start capacitor or tracking the output of the other channel or supply. When the device is conﬁgured to soft-start by itself, an external soft-start capacitor should be connected to the TRCKSS pin. A softstart current of 1.2μA is to charge the soft-start capacitor CSS.
PAGE 21
LTC3828 APPLICATIONS INFORMATION frequency fO. A voltage applied to the PLLFLTR pin of 1.2V corresponds to a frequency of approximately 400kHz. The nominal operating frequency range of the IC is 260kHz to 550kHz. The phase detector used is an edge sensitive digital type which provides zero degrees phase shift between the external and internal oscillators. This type of phase detector will not lock up on input frequencies close to the harmonics of the VCO center frequency.
PAGE 22
LTC3828 APPLICATIONS INFORMATION The resistive load reduces the DC loop gain while maintaining the linear control range of the error ampliﬁer. The maximum output voltage deviation can theoretically be reduced to half or alternatively the amount of output capacitance can be reduced for a particular application. A complete explanation is included in Design Solutions 10. (See www.linear.
PAGE 23
LTC3828 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ΔILOAD (ESR), where ESR is the effective series resistance of COUT .
PAGE 24
LTC3828 APPLICATIONS INFORMATION during load-dump. Note that the transient suppressor should not conduct during double-battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LTC3828 have a maximum input voltage of 30V, most applications will also be limited to 30V by the MOSFET BVDSS. 50A IPK RATING 12V VIN LTC3828 TRANSIENT VOLTAGE SUPPRESSOR GENERAL INSTRUMENT 1.5KA24A 3828 F11a Figure 11a.
PAGE 25
LTC3828 APPLICATIONS INFORMATION 1.8V 2 (5) [1+ (0.005)(50°C – 25°C )] • 22V (0.035Ω) + (22V)2 ⎛⎜⎝ 52A ⎞⎟⎠ (4Ω)(215pF ) • PMAIN = 1 ⎤ ⎡ 1 ⎢ 5 – 2.3 + 2.3 ⎥(300kHz ) = 332mW ⎣ ⎦ A short-circuit to ground will result in a folded back current of: ISC = 25mV 1 ⎛ 120ns(22V)⎞ – ⎜ ⎟ = 2.1A 0.01Ω 2 ⎝ 3.3μH ⎠ with a typical value of RDS(ON) and δ = (0.005/°C)(20) = 0.1. The resulting power dissipated in the bottom MOSFET is: 22V – 1.8V 2 2.1A ) (1.125)(0.
PAGE 26
LTC3828 APPLICATIONS INFORMATION RPU 1 2 3 R1 R2 4 5 6 7 fIN 8 9 10 11 12 13 R3 R4 14 TRCKSS1 CLKOUT 28 VPULL_UP <5.5V PGOOD L1 RSENSE1 VOUT1 27 PGOOD LTC3828 26 BOOST1 SENSE1+ ITH1 SENSE1– TG1 VOSENSE1 SW1 PLLFLTR VIN RUN1 FCB/PLLIN INTVCC PGND SGND BG1 TRCKSS2 BG2 SENSE2– SW2 SENSE2+ TG2 ITH2 VOSENSE2 BOOST2 RUN2 25 M1 CB1 M2 D1 24 23 RIN 22 CVIN 21 VIN COUT1 CIN 20 CINTVCC COUT2 19 18 17 CB2 M3 M4 D2 16 15 L2 RSENSE2 VOUT2 3828 F12 Figure 12.
PAGE 27
LTC3828 APPLICATIONS INFORMATION SW1 L1 D1 RSENSE1 COUT1 VOUT1 + RL1 CERAMIC VIN RIN CIN + SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 L2 RSENSE2 COUT2 VOUT2 + RL2 CERAMIC 3728 F11 Figure 13.
PAGE 28
LTC3828 APPLICATIONS INFORMATION PC Board Layout Debugging Start with one controller on at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage as well. Check for proper performance over the operating voltage and current range expected in the application.
PAGE 29
LTC3828 APPLICATIONS INFORMATION 10Ω 2nF RPU 33pF 1 15k R1 R2 20k 1% 105k 1% 2 4 6 7 8 9 30k 158k 10 11 33pF 1000pF 12 15k R3 R4 20k 1% 63.4k 1% CLKOUT L1 4.7μH PGOOD 13 14 ITH1 SENSE1– TG1 VOSENSE1 SW1 PLLFLTR VIN RUN1 FCB/PLLIN INTVCC PGND SGND BG1 TRCKSS2 BG2 SENSE2– SW2 SENSE2+ TG2 BOOST2 ITH2 VOSENSE2 RUN2 25 CB1 0.22μF M1 M2 RIN 10Ω 23 CVIN 0.1μF 1mF D1 1μF CERAMIC 24 22 COUT1 150μF 6.3V CIN 22μF 50V 21 CINTVCC 4.
PAGE 30
LTC3828 APPLICATIONS INFORMATION 10Ω 2nF RPU 33pF 1 R1 20k 1% 15k 2 R2 17.4k 1% 4 1000pF 0.1μF 10k 6 7 fIN 500kHz 8 9 30k 26.1k 10 11 33pF 12 1000pF 4.7k R3 R4 20k 1% 63.4k 1% CLKOUT L1 0.68μH PGOOD 13 14 ITH1 SENSE1– TG1 VOSENSE1 SW1 PLLFLTR VIN RUN1 FCB/PLLIN INTVCC PGND SGND BG1 TRCKSS2 BG2 SENSE2– SW2 SENSE2+ TG2 ITH2 VOSENSE2 BOOST2 RUN2 25 CB1 0.22μF M1 M2 RIN 10Ω 23 CVIN 0.1μF 1μF D1 1μF CERAMIC 24 22 COUT1 330μF 2.5V CIN 22μF 16V 21 CINTVCC 4.
PAGE 31
LTC3828 PACKAGE DESCRIPTION G Package 28-Lead Plastic SSOP (5.3mm) (Reference LTC DWG # 05-08-1640) 9.90 – 10.50* (.390 – .413) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1.25 ±0.12 7.8 – 8.2 5.3 – 5.7 0.42 ±0.03 7.40 – 8.20 (.291 – .323) 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 RECOMMENDED SOLDER PAD LAYOUT 2.0 (.079) MAX 5.00 – 5.60** (.197 – .221) 0° – 8° 0.09 – 0.25 (.0035 – .010) 0.65 (.0256) BSC 0.55 – 0.95 (.022 – .037) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
PAGE 32
LTC3828 TYPICAL APPLICATION IIN 12VIN CIN I1 PHASMD OPEN TG1 U1 TG2 LTC3729 90° 0° IIN* BUCK: 2.5V/15A 180° BUCK: 2.5V/15A 2.5VO/30A I2 CLKOUT I2 I3 I3 TG1 U2 TG2 LTC3828 90° 90° BUCK: 1.5V/15A 270° BUCK: 1.8V/15A 1.5VO/15A 1.8VO/15A I4 *INPUT RIPPLE CURRENT CANCELLATION INCREASES THE RIPPLE FREQUENCY AND REDUCES THE RMS INPUT RIPPLE CURRENT THUS, SAVING INPUT CAPACITORS I4 PLLIN I1 3828 F16 Figure 16.