SEMTECH SC4609 Technical data

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High Efficiency Synchronous Buck

POWER MANAGEMENT

Description Features

SC4609

Low Input, MHz Operation,

The SC4609 is a voltage mode step down (buck) regulator controller that provides accurate high efficiency power conversion from an input supply range of 2.7V to 5.5V. A high level of integration reduces external component count, and makes it suitable for low voltage applications where cost, size and efficiency are critical. The SC4609 is capable of producing an output voltage as low as 0.5V.

The SC4609 drives external, N-channel MOSFETs with a peak gate current of 1A. A non-overlap protection is provided for the gate drive signals to prevent shoot through of the MOSFET pair. The SC4609 features lossless current sensing of the voltage drop across the drain to source resistance of the high side MOSFET during its conduction period. Its switching frequency can be programmed up to 1MHz.

The quiescent supply current in sleep mode is typically lower than 10

µA. A external soft start is provided to pre-

vent output voltage overshoot during start-up.

The SC4609 is an ideal choice for converting 3.3V, 5V or other low input supply voltages. It’s available in 12 pin MLP package.

 Asynchronous start up  Programmable switching frequency up to 1MHz  BiCMOS voltage mode PWM controller  2.7V to 5.5V input voltage range  Output voltage as low as 0.5V  +/-1% reference accuracy  Sleep mode (Icc = 10µA typ)  Adjustable lossless short circuit current limiting  Combination pulse by pulse & hiccup mode

current limit

 High efficiency synchronous switching  1A peak current driver  External soft start  12-pin MLP Lead-free package, fully WEEE and RoHS

compliant

Applications

 Distributed power architecture  Servers/workstations  Local microprocessor core power supplies  DSP and I/O power supplies  Battery-powered applications  Telecommunications equipment  Data processing applications

Typical Application Circuit

R13

2.2n

14.3k

BST

4.7u

C16

560pF

*External components can be modified to provide a Vout as low as 0.5V

BST

VCC

ISET

COMP

FSET

VSENSE

180p

C17

PHASE

SC4609

DRVH

DRVL

PGND

AGND

Css

22u

Vin=2.7V - 5.5V

C10

220u

1.8u

220u

330u

M11

C13

C14

22u

Vout=1.5V (as low as 0.5V*) / 12A

22u

4.7n

200

10k

4.99k

Revision: February 8, 2006

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SC4609

POWER MANAGEMENT

Absolute Maximum Ratings

Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied.

retemaraPlobmySmumixaMstinU

)6V

V(egatloVylppuS

DNGP 3.0±V

stnerruC)LVRD,HVRD(srevirDtuptuO

52.0±A

suounitnoC

kaeP

00.1±A

)SS,TESI,TESF,PMOC,ESNESV(stupnI 6ot3.0-V

TSB 21V

ESAHP 6ot3.0-V

sn05<esluptesluPESAHP 7ot2-V

egnaRerutarepmeTtneibmAgnitarepOT

egnaRerutarepmeTegarotST

erutarepmeTnoitcnuJmumixaMT

s04-01,erutarepmeTwolfeRRIkaePT

GTS

GKP

58+ot04-C°

051+ot56-C°

051+C°

062C°

)ledoMydoBnamuH(gnitaRDSEDSE4Vk

All voltages with respect to AGND. Currents are positive into, negative out of the specified terminal.

Electrical Characteristics

Unless otherwise specified, VCC = 3.3V, CT = 270pF, TA = -40°C to 85°C, TA=T

retemaraPsnoitidnoCtseTniMpyTxaMtinU

Note: (1). Guaranteed by design.

llarevO

egatloVylppuS 5.5V

peelS,tnerruCylppuSV0=TESF0151Aµ

gnitarepO,tnerruCylppuSV

dlohserhTno-nruTCCVT

=V5.5257.3Am

C°58otC°04-=7.2V

siseretsyHffo-nruTCCV 053Vm

reifilpmArorrE

egatloVtupnI

C°52=594.05.0505.0

)ecnerefeRlanretnI(

tnerruCsaiBESNESVV

)1(

niaGpooLnepO

)1(

htdiwdnaBniaGytinU

PMOC

T,V5.5-V7.2=

V5.0=002An

ESNES

C°52=5294.05.05705.0

C°58otC°04-=5294.05705.0

V5.2ot5.0=09Bd

8zHM

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Electrical Characteristics (Cont.)

Unless otherwise specified, VCC = 3.3V, CT = 270pF, TA = -40°C to 85°C, TA=T

retemaraPsnoitidnoCtseTniMpyTxaMtinU

).tnoC(reifilpmArorrE

SC4609

)1(

etaRwelS

hgiHTUOVI

woLTUOVI

PMOC

Am5.5-=V

Am5.5=3.054.0

5.0-VCC3.0-V

4.2sµ/V

rotallicsO

ycaruccAlaitinIT

ytilibatSegatloVT

tneiciffeoCerutarepmeTT

)1(

ycneuqerFnoitarepOmuminiM

)1(

ycneuqerFnoitarepOmumixaM

V,C°52=

C°52=525575526zHk

V5.5otV7.2=50.0V/%

C°58otC°04-=20.0C°/%

05zHk

yellaVotkaePpmaR 1V

egatloVkaePpmaR 3.1V

egatloVyellaVpmaR 3.0V

timiLtnerruC,tratStfoS,peelS

dlohserhTpeelSTESFtaderusaeM57Vm

tnerruCsaiBtupnIpeelSV

)1(

emiTtratStfoSelbammargorP

V0=1-Aµ

CNYS

Fn02=C57.1sm

M1zH

tnerruCegrahCtratStfoST

tnerruCsaiBTESIT

TESIfotneiciffeoCerutarepmeT 82.0C°/%

)1(

emiTknalBtimiLtnerruC

evirDetaG

emiTFFOmuminiMHVRDT

)HVRD(ecruoSkaePI,V3.3=sgV

)HVRD(kniSkaePI,V3.3=sgV

)1(

)LVRD(ecruoSkaeP

)LVRD(kniSkaePI,V3.3=sgV

emiTesiRtuptuOC,V3.3=sgV

emiTllaFtuptuOC,V3.3=sgV

)1(

palrevO-noNmuminiM

C°52=57.5-Aµ

C°52=54-05-55-Aµ

031sn

V,C°52=

I,V3.3=sgV

0=061002sn

ESNES

ECRUOS

KNIS

ECRUOS

KNIS

TUO

Am001=7.2

Am001=8.1

Am001=2.2

Am001=5.1

Fn7.4=53sn

Fn7.4=72sn

Ω Ω Ω Ω

04sn

3 2006 Semtech Corp. www.semtech.com

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SC4609

Pin Configuration

BST

1VCC

2ISET

3COMP

FSET5VSENSE6SS

(MLP12, 4x4)

Pin Descriptions

#niPemaNniPnoitcnuFniP

1CCV RSE/LSEwolFµ7.4ot1.0ahtiwDNGotnipsihtssapyB.CIehtrofliarylppusevitisoP

TOP VIEW

DRVH PHASE

11 10

9 DRVL

8 PGND

7 AGND

Ordering Information

)1(

rebmuNtraP

)2(

TRTLM9064CS

BVE9064CSdraoBnoitaulavE

Notes: (1) Only available in tape and reel packaging. A reel contains 3000 devices. (2) Lead free product. This product is fully WEEE and

RoHS compliant.

.roticapaccimarec

eciveD

21-PLM

2TESI ehtsesu9064CSehT.TEFSOMedishgihehtnitnerructimilotdesusinipTESIehT

3PMOC detrevnisituptuosihttaegatlovehT.reifilpmarorreegatlovehtfotuptuoehtsisihT

4TESF gnimitlanretxenahguorhtycneuqerfrotallicsoMWPehtstesotdesusinipTESFehT

5ESNESV egatlovtuptuoehtsasevresdnareifilpmaegatlovehtfotupnignitrevniehtsinipsihT

Vehtssorcaegatlov

timiltnerrucehT.timiltnerrucehttesotredronisnipTESIdna

tiucriCnoitacilppAlacipyTehtni3R(rotsiserlanretxenafoeulavehtybtessidlohserht

esnesehtssorcapordegatlovehtgnirapmocybdemrofrepsignitimiltnerruC.)margaiD

edishgihehtfoecnatsiserecruosotniardehtssorcapordegatlovehthtiwrotsiser

otniardehtssorcapordegatlovehT.doirepnoitcudnocs’TEFSOMehtgnirudTEFSOM

VehtmorfdeniatbosiTEFSOMedishgihehtfoecnatsiserecruos

.nipESAHPdna

gal-daelA.rotarapmocMWPehtfotupnignitrevni-nonehtotdetcennocdnayllanretni

retlifCLelopowtehtrofsetasnepmocnipESNESVehtotnipPMOCehtmorfkrowten

redronideriuqersikrowtengal-daelehT.lortnocedomegatlovottnerehniscitsiretcarahc

.poollortnocedomegatlovehtfoecnamrofrepcimanydehtezimitpoot

dellupsiTESFehtnehW.nipDNGehtotnipTESFehtmorfdetcennocsitahtroticapac

sinoitarepoedompeelS.dekovnisinoitarepoedompeelssti,Vm57wolebdlehdna

tnerrucylppuslacipytehT.Vm57wolebegatlovaotnipTESFehtgnipmalcybdekovni

gnicalpybedomsuonorhcnysnidetarepoebnac9064CSehT.Aµ01siedompeelsgnirud

ehtfolanimretrehtoehT.dnuorgdnaroticapacgnimitehtneewtebseiresnirotsisera

.nipTESFehtotdetcennocniamerlliwroticapacgnimit

eulavecnereferlanretninaotderapmocsiESNESV.retrevnockcuBehtroftniopkcabdeef

.derisedsiV5.0fotuptuonanehwegatlovtuptuoehtotderiwdrahsiESNESV.V5.0fo

lacipyTehtni9Rdna7R(yrassecensikrowtenredividrotsisera,segatlovtuptuorehgihroF

.)margaiDtiucriCnoitacilppA

6SS tnednepednisiemittratstfosehT.emittratstfosehtstesdnuorgotroticapacA.tratstfoS

••=

.C105.87SS

lanretxeehtsiCerehWsadenifedsidnaycneuqerfgnihctiwsfo

.dnocesniemittratstfosdnaFnniroticapac

4 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Pin Descriptions (Cont.)

#niPemaNniPnoitcnuFniP

7DNGA.dnuorggolanA

8DNGP.dnuorgrewoP

9LVRD srevirdtuptuoehT.TEFSOM)reifitcersuonorhcnys(ediswolehtfoetagehtsevirdLVRD

01ESAHP ehtsesu9064CSehT.TEFSOMedishgihehtnitnerructimilotdesusinipESAHPehT

11HVRD detarerasrevirdtuptuoehT.TEFSOM)hctiwsniam(edishgihehtfoetagehtsevirdHVRD

SC4609

otslangisevirdyratnemelpmocsedivorpyrtiucricMWPehT.stnerruckaepA1rofdetarera

ybdetneverpsisTEFSOMlanretxeehtfonoitcudnocssorcehT.segatstuptuoeht

emitahtiwnoitcnujnocniriapTEFSOMehtfosniprevirdehtnoegatlovehtgnirotinom

.scitsiretcarahcffo-nrutTEFrofdezimitpoyaled

Vehtssorcaegatlov

VehtmorfdeniatbosiTEFSOMedishgihehtfoecnatsiserecruos

.scitsiretcarahcffo-nrutTEFrofdezimitpo

timiltnerrucehT.timiltnerrucehttesotredroninipTESIdna

tiucriCnoitacilppAlacipyTehtni3R(rotsiserlanretxenafoeulavehtybtessidlohserht

esnesehtssorcapordegatlovehtgnirapmocybdemrofrepsignitimiltnerruC.)margaiD

edishgihehtfoecnatsiserecruosotniardehtssorcapordegatlovehthtiwrotsiser

otniardehtssorcapordegatlovehT.doirepnoitcudnocs’TEFSOMehtgnirudTEFSOM

.nipESAHPdna

ehtotslangisevirdyratnemelpmocsedivorpyrtiucricMWPehT.stnerruckaepA1rof

gnirotinomybdetneverpsisTEFSOMlanretxeehtfonoitcudnocssorcehT.segatstuptuo

yaledemitahtiwnoitcnujnocniriapTEFSOMehtfosniprevirdehtnoegatloveht

21TSB otstcennocTSB.TEFSOMedishgihlennahC-NnaevirdotretrevnocehtselbanenipsihT

DAPLAMREHT detcennoctoN.saivelpitlumgnisuenalpdnuorgottcennoC.sesoprupgniknistaehrofdaP

aotegatlovnipTSBehtstsoobtiucricpmupegrahcehT.tiucricpmupegrahclanretxeeht

.TEFSOMedishgihehtfoetagehtgnivirdroflevelegatlovecruos-ot-etagtneiciffus

.yllanretni

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POWER MANAGEMENT

Block Diagram

SC4609

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POWER MANAGEMENT

Application Information

Enable

The SC4609 is enabled by applying a voltage greater than

2.7 volts to the VCC pin. The SC4609 is disabled when VCC falls below 2.35 volts or when sleep mode operation is invoked by clamping the FSET pin to a voltage below 75mV. 10µA is the typical current drawn through the VCC pin during sleep mode. During the sleep mode, the high side and low side MOSFETs are turned off and the internal soft start voltage is held low.

Oscillator

SC4609

The maximum frequency of the external clock signal can be higher than the natural switching frequency by about 10%.

FSET

External

External Clock

Clock Signal

Signal

56pF

FSET

100

SYNC

SC4609

The FSET pin is used to set the PWM oscillator frequency through an external timing capacitor that is connected from the FSET pin to the GND pin. The resulting ramp waveform ion the FSET pin is a triangle at the PWM frequency with a peak voltage of 1.3V and a valley voltage of 0.3V. 200ns minimum OFF time for the top switch allows the bootstrap capacitor to be charged during each cycle. The capacitor tolerance adds to the accuracy of the oscillator frequency. The approximate operating frequency and soft start time are both determined by the value of the external timing capacitor as shown in Table

gnimiTlanretxE

)Fp(eulaVroticapaC

0210001

072575

074053

065592

)zHk(ycneuqerF

Table 1. Operating Frequency value Based on the

Value of the External Timing Capacitor Placed Across

the FSET and GND Pins

Synchronous mode operation is invoked by using a sig-

nal from an external clock. A low value resistor (100Ω

typical) must be inserted in series with the timing capacitor between the timing capacitor and the GND pin. The other terminal of the timing capacitor will remain connected to the FSET pin. The transformed external clock signal is then connected to the junction of the external timing capacitor and the added resistor R

as shown

SYNC

in Figure 1.

Figure 1

UVLO

When the FSET pin is not pulled and held below 75mV, the voltage on the Vcc pin determines the operation of the SC4609. As Vcc increases during start up, the UVLO block senses Vcc and keeps the high side and low side MOSFETs off and the internal soft start voltage low until Vcc reaches 2.7V. If no faults are present, the SC4609 will initiate a soft start when Vcc exceeds 2.7V. A hysteresis (350mV) in the UVLO comparator provides noise immunity during its start up.

Soft Start

The soft start function is required for step down controllers to prevent excess inrush current through the DC bus during start up. Generally this can be done by sourcing a controlled current into a timing capacitor and then using the voltage across this capacitor to slowly ramp up the error amp reference. The closed loop creates narrow width driver pulses while the output voltage is low and allows these pulses to increase to their steady state duty cycle as the output voltage reaches its regulated value. With this, the inrush current from the input side is controlled. The duration of the soft start in the SC4609 is controlled by an external capacitor. SS, the startup time is difined as:

••=

C105.87SS

where, C is the value of the external capacitor in nF, and SS is the startup time in second.

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Application Information (Cont.)

SC4609

Over Current Protection

The SC4609 detects over current conditions by sensing the voltage across the drain-to-source of the high side MOSFET. The SC4609 determines the high side MOSFET current level by sensing the drain-to-source conduction voltage across the high side MOSFET via the V

(see

the Typical Application Circuit on page 1) and PHASE pin during the high side MOSFET’s conduction period. This voltage value is then compared internally to a user programmed current limit threshold. Note that user should place Kelvin sensing connections directly from the high side MOSFET source to the PHASE pin.

The current limit threshold is programmed by the user based on the RDS(on) of the high side MOSFET and the value of the external set resistor RSET (where RSET is represented by R3 in the applications schematics of this document). The SC4609 uses an internal current source to pull a 50µA current from the input voltage to the ISET pin through external resistor RSET. The current limit threshold resistor (RSET) value is calculated using the following equation:

⋅

)ON(DSMAX

A50

The R

SET

sensing used in the SC4609 has an addi-

DS(ON)

tional feature that enhances the performance of the over current protection. Because the R

has a positive

DS(ON)

temperature coefficient, the 50µA current source has a positive coefficient of about 0.28%/C° providing first order correction for current sensing vs temperature. This compensation depends on the high amount of thermal transferring that typically exists between the high side NMOSFET and the SC4609 due to the compact layout of the power supply.

When the converter detects an over current condition (I > I

) as shown in Figure 2, the first action the SC4609

MAX

takes is to enter the cycle by cycle protection mode (Point B to Point C), which responds to minor over current cases. Then the output voltage is monitored. If the over current and low output voltage (set at 70% of nominal output voltage) occur at the same time, the Hiccup mode operation (Point C to Point D) of the SC4609 is invoked and the internal soft start capacitor is discharged. This is like a typical soft start cycle:

−

0.7

⋅6.0

−

nomO

MAX

Figure 2. Over current protection characteristic of SC4609

Power MOSFET Drivers

The SC4609 has two drivers which are optimized for driving external power N-Channel MOSFETs.. The driver block consists two 1 Amp drivers. DRVH drives the high side N-MOSFET (main switch), and DRVL drives the low side N-MOSFET (synchronous rectifier transistor). The output drivers also have gate drive non-overlap mechanism that provides a dead time between DRVH and DRVL transitions to avoid potential shoot through problems in the external MOSFETs. By using the proper design and the appropriate MOSFETs, the SC4609 is capable of driving a converter with up to 12A of output

the delay from the

current. As shown in Figure 3, top MOSFET off to the bottom MOSFET on is adaptive by detecting the voltage of the phase node.

td1

td2, the delay

from the bottom MOSFET off to the top MOSFET on is fixed, is 40ns for the SC4609. This control scheme guarantees avoidance of cross conduction or shoot through between the upper and lower MOSFETs and also minimizes the conduction loss in the body diode of the bottom MOSFET for high efficiency applications.

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POWER MANAGEMENT

SC4609

Application Information (Cont.)

TOP MOSFET Gate Drive

BOTTOM MOSFET Gate Drive

Ground

Phase node

Figure 3. Timing Waveforms for Gate Drives and Phase Node

Inductor Selection

The factors for selecting the inductor include its cost, efficiency, size and EMI. For a typical SC4609 application, the inductor selection is mainly based on its value, saturation current and DC resistance. Increasing the inductor value will decrease the ripple level of the output voltage while the output transient response will be degraded. Low value inductors offer small size and fast transient responses while they cause large ripple currents, poor efficiencies and more output capacitance to smooth out the large ripple currents. The inductor should be able to handle the peak current without saturating and its copper resistance in the winding should be as low as possible to minimize its resistive power loss. A good tradeoff among its size, loss and cost is to set the inductor ripple current to be within 15% to 30% of the maximum output current. The inductor value can be determined according to its operating point and the switching frequency as follows:

)VV(V

−⋅

OUTINOUT

⋅∆⋅⋅

IIfV

OMAXsIN

Where: fs = switching frequency and

∆I = ratio of the peak to peak inductor current to the

maximum output load current.

The peak to peak inductor current is:

Ground

−

OMAXPEAK

The power loss for the inductor includes its core loss and copper loss. If possible, the winding resistance should be minimized to reduce inductor’s copper loss. The core loss can be found in the manufacturer’s datasheet. The inductor’ copper loss can be estimated as follows:

COPPER

LRMS

WINDING

RIP ⋅=

Where: I

is the RMS current in the inductor. This current can

LRMS

be calculated as follow is:

OMAXLRMS

1II ∆⋅+⋅=

Output Capacitor Selection

Basically there are two major factors to consider in selecting the type and quantity of the output capacitors. The first one is the required ESR (Equivalent Series Resistance) which should be low enough to reduce the voltage deviation from its nominal one during its load changes. The second one is the required capacitance, which should be high enough to hold up the output voltage. Before the SC4609 regulates the inductor current to a new value during a load transient, the output capacitor delivers all the additional current needed by the load. The ESR and ESL of the output capacitor, the loop parasitic inductance between the output capacitor and the load combined with inductor ripple current are all major contributors to the output voltage ripple. Surface mount speciality polymer aluminum electrolytic chip capacitors in UE series from Panasonic provide low ESR and reduce the total capacitance required for a fast transient response. POSCAP from Sanyo is a solid electrolytic chip capacitor that has a low ESR and good performance for high frequency with a low profile and high capacitance. Above mentioned capacitors are recommended to use in SC4609 application.

•∆=

III

−

OMAXpp

After the required inductor value is selected, the proper selection of the core material is based on the peak inductor current and efficiency requirements. The core must be able to handle the peak inductor current I

PEAK

without saturation and produce low core loss during the high frequency operation is:

Input Capacitor Selection

The input capacitor selection is based on its ripple current level, required capacitance and voltage rating. This capacitor must be able to provide the ripple current by the switching actions. For the continuous conduction mode, the RMS value of the input capacitor can be calculated from:

9 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Application Information (Cont.)

CIN

⋅=

OMAX

)RMS(

−⋅

SC4609

)VV(V

OUTINOUT

Where:

= the boost current and

= discharge ripple voltage.

This current gives the capacitor’s power loss as follows:

CIN

RIP ⋅=

)RMS(CIN

)ESR(CIN

This capacitor’s RMS loss can be a significant part of the total loss in the converter and reduce the overall converter efficiency. The input ripple voltage mainly depends on the input capacitor’s ESR and its capacitance for a given load, input voltage and output voltage. Assuming that the input current of the converter is constant, the required input capacitance for a given voltage ripple can be calculated by:

−⋅

⋅=

OMAXIN

)D1(D

⋅−∆⋅

)RIV(fs

CINOMAXI

)ESR(

Where: D = VO/VI , duty ratio and

∆V

= the given input voltage ripple.

Because the input capacitor is exposed to the large surge current, attention is needed for the input capacitor. If tantalum capacitors are used at the input side of the converter, one needs to ensure that the RMS and surge ratings are not exceeded. For generic tantalum capacitors, it is wise to derate their voltage ratings at a ratio of 2 to protect these input capacitors.

Boost Capacitor Selection

The boost capacitor selection is based on its discharge ripple voltage, worst case conduction time and boost current. The worst case conduction time Tw can be estimated as follows:

Tw ⋅=

max

Where: fs = the switching frequency and Dmax = maximum duty ratio.

The required minimum capacitance for boost capacitor will be:

C ⋅=

boost

With fs = 300kH, VD=0.3V and IB=50mA, the required capacitance for the boost capacitor is:

boost

max

05.0

3.0

k300

=⋅⋅=⋅⋅=

nF52895.0

Power MOSFET Selection

The SC4609 can drive an N-MOSFET at the high side and an N-MOSFET synchronous rectifier at the low side. The use of the high side N-MOSFET will significantly reduce its conduction loss for high current. For the top MOSFET, its total power loss includes its conduction loss, switching loss, gate charge loss, output capacitance loss and the loss related to the reverse recovery of the bottom diode, shown as follows:

⋅⋅

fVI

TOTAL_TOP

RIP

RMS_TOP

+⋅=

ON_TOP

GATE

sIPEAK_TOP

⋅

fV)QQ(fVQ)QQ(

⋅⋅++⋅⋅++

sIrrOSSsGATEGT2GSGD

Where: RG = gate drive resistor, QGD = the gate to drain charge of the top MOSFET, Q

= the gate to source charge of the top MOSFET,

GS2

= the total gate charge of the top MOSFET,

= the output charge of the top MOSFET and

OSS

Qrr = the reverse recovery charge of the bottom diode.

For the top MOSFET, it experiences high current and high voltage overlap during each on/off transition. But for the bottom MOSFET, its switching voltage is the bottom diode’s forward drop during its on/off transition. So the switching loss for the bottom MOSFET is negligible. Its total power loss can be determined by:

TOTAL_BOT

RMS_BOT

V_IfVQRIP ⋅+⋅⋅+⋅=

FAVGDsGATEGBON_BOT

Where: Q

= the total gate charge of the bottom MOSFET and

VF = the forward voltage drop of the bottom diode.

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Application Information (Cont.)

For a low voltage and high output current application such as the 3.3V/1.5V@12A case, the conduction loss is often dominant and selecting low R ticeably improve the efficiency of the converter even though they give higher switching losses.

The gate charge loss portion of the top/bottom MOSFET’s total power loss is derived from the SC4609. This gate charge loss is based on certain operating conditions (fs, V

, and IO).

GATE

The thermal estimations have to be done for both MOSFETs to make sure that their junction temperatures do not exceed their thermal ratings according to their total power losses P thermal resistance

, ambient temperature TA and their

TOTAL

RθJA

as follows:

MOSFETs will no-

DS(ON)

SC4609

Figure 4. Compensation network provides 3 poles and 2 zeros.

For voltage mode step down applications as shown in Figure 4, the power stage transfer function is:

TOTAL

A(max)J

Loop Compensation Design

For a DC/DC converter, it is usually required that the converter has a loop gain of a high cross-over frequency for fast load response, high DC and low frequency gain for low steady state error, and enough phase margin for its operating stability. Often one can not have all these properties at the same time. The purpose of the loop compensation is to arrange the poles and zeros of the compensation network to meet the requirements for a specific application.

The SC4609 has an internal error amplifier and requires the compensation network to connect among the COMP pin and VSENSE pin, GND, and the output as shown in Figure 4. The compensation network includes C1, C2, R1, R7, R8 and C9. R9 is used to program the output voltage according to

+⋅=

)

1(5.0V

⋅

V)s(G

IVD

CLs

Where: R = load resistance and RC = C4’s ESR.

The compensation network will have the characteristic as follows:

⋅

+⋅

COMP

)s(G

1ZI

⋅

Where

=ω

+⋅

)CC(R

217

=ω

⋅

=ω

11 2006 Semtech Corp. www.semtech.com

⋅+

C)RR(

987

CCR

⋅⋅

211

POWER MANAGEMENT

Application Information (Cont.)

SC4609

=ω

⋅

After the compensation, the converter will have the following loop gain:

⋅ω⋅

=⋅⋅=

)s(G)s(GG)s(T

VDCOMPPWM

1sV

⋅

+⋅

⋅

CLs

Where: G

= PWM gain

PWM

VM = 1.0V, ramp peak to valley voltage of SC4609

The design guidelines for the SC4609 applications are as following:

1. Set the loop gain crossover corner frequency ω for given switching corner frequency ω

= 2pfs,

2. Place an integrator at the origin to increase DC and low frequency gains.

3. Select ωZ1 and ω

to damp the peaking and the loop gain has a

such that they are placed near

-20dB/dec rate to go across the 0dB line for obtaining a wide bandwidth.

4. Cancel the zero from C4’s ESR by a compensator

pole ωP1 (ωP1 = ω

= 1/( RCC4)).

ESR

5. Place a high frequency compensator pole ωp2 (ωp = πf

) to get the maximum attenuation of the switch-

ing ripple and high frequency noise with the adequate

phase lag at ω

The compensated loop gain will be as given in Figure 5:

Layout Guidelines

In order to achieve optimal electrical, thermal and noise performance for high frequency converters, special attention must be paid to the PCB layouts. The goal of layout optimization is to identify the high di/dt loops and minimize them. The following guideline should be used to ensure proper functions of the converters.

1. A ground plane is recommended to minimize noises and copper losses, and maximize heat dissipation.

2. Start the PCB layout by placing the power components first. Arrange the power circuit to achieve a clean power flow route. Put all the connections on one side of the PCB with wide copper filled areas if possible.

3. The Vcc bypass capacitor should be placed next to

the Vcc and GND pins.

4. The trace connecting the feedback resistors to the output should be short, direct and far away from the noise sources such as switching node and switching components.

5. Minimize the traces between DRVH/DRVL and the gates of the MOSFETs to reduce their impedance to drive the MOSFETs.

6. Minimize the loop including input capacitors, top/bottom MOSFETs. This loop passes high di/dt current.

Make sure the trace width is wide enough to reduce copper losses in this loop.

7. ISET and PHASE connections to the top MOSFET for current sensing must use Kelvin connections.

8. Maximize the trace width of the loop connecting the inductor, bottom MOSFET and the output capacitors.

Gvd

0dB

Power stage

Loop gain

ESR

Figure 5. Asymptotic diagrams of power stage and its loop gain

9. Connect the ground of the feedback divider and the compensation components directly to the GND pin of the SC4609 by using a separate ground trace. Then connect this pin to the ground of the output capacitor as close as possible

12 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Application Information (Cont.)

Design Example 1. 3V to1.5V @10A application with SC4609

C17

R13

4.7u

C16

R13

470pF

BST

VCC

ISET

COMP

FSET

VSENSE

SC4609

DRVH

PHASE

DRVL

PGND

AGND

Css

22n

1.8n

2.2n

14.3k

C10

220u

2.3u

330u

Vin=3V - 5.5V

C13

C14

C13

C14

22u

Vout = 1.5V/10A

8.2n

5.76k

107

SC4609

2.87k

13 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Bill of Materials

metIytQecnerefeReulaVrerutcafunaM/.oNtraP

111CFn8.1

212CFn2.2

3171CFu1

44 41C,31C,5C,4C6021,Fu22M622J0R5X5223C:N/PKDT

517C0782,Fu033LM033BPT6:N/PoynaS

619CFn2.8

7161CFp074

812D1TL0250RBM1TL0250RBM:N/PimeSNO

SC4609

911LHu3.2

012 2M,1M8-OS,kcaprewoPPD2887iS:N/PyahsiV

1111RK3.41

2113RK33.1

3117RK67.5

4118R701

5119RK78.2

61131R1

7113C5080,Fu7.4

81101C0782,Fu022LM022BPT6:N/PoynaS

911ssCFn22

0211U9064CSTRTLMI9064CS:N/PhcetmeS

.egakcap3060htiwnoisicerp%1evahsrotsiserlla,deificepssselnU

%02-/+erasroticapaclladna%1-/+erasrotsiseR

cinortcelErepooC

3R2-1CH:N/P

14 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

PCB Layout

SC4609

COMPONENT SIDE (TOP)

COMPONENT SIDE (BOTTOM)

15 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

PCB Layout (Cont.)

SC4609

(TOP)

(BOTTOM)

(INTER LAYER 1)

(INTER LAYER 2)

16 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Outline Drawing - MLP-12

PIN 1

INDICATOR

(LASER MARK)

aaa C

A D

DIM

SEATING PLANE

DIMENSIONS

INCHES

NOM

MIN

.031

A A1 A2

D D1

E E1

aaa bbb

.000

(.008)

.012

.010

.074

.085 .153 .157 .161 3.90 4.00 4.10 .074

.085

.031 BSC

.018

.022

.004 0.10

MILLIMETERS

MINMAX NOM

.040

0.80

.002

0.00

.014

0.25

.089

1.90

.089

1.90

.026

0.45

(0.20)

0.30

2.15

0.80 BSC

0.55 12

0.08.003

SC4609

MAX

1.00

0.05

0.35

4.104.003.90.161.157.153

2.25

0.65

LxN

E/2

bxN

bbb C A B

D/2

NOTES:

CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).

COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.

17 2006 Semtech Corp. www.semtech.com

POWER MANAGEMENT

Land Pattern - MLP-12

2x (C)

SC4609

DIMENSIONS

DIM

2x G

2x Z

G H

K P X Y Z

(.148)

.106 .091 .091 .031 .016 .041 .189

MILLIMETERSINCHES

(3.75)

2.70

2.30

0.80

0.40

1.05

4.80

NOTES:

THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.

Contact Information

Semtech Corporation

Power Management Products Division

200 Flynn Road, Camarillo, CA 93012

Phone: (805)498-2111 FAX (805)498-3804

18 2006 Semtech Corp. www.semtech.com

SEMTECH SC4609 Technical data

Specifications and Main Features

Frequently Asked Questions

User Manual