Datasheet US3036CS Datasheet (UNISEM)

Page 1

FEATURESFEATURES

8 pin SOIC Switching Controller with HICCUP Current Limiting Reduces Diode Power Dissipation to Less than 1% of Normal Operation Soft Start Capacitor allows for smooth Output Voltage ramp up On board MOSFET driver Fastest transient response of any controller method. ( 0 to 100% Duty Cycle in 100 nS ) 1% internal voltage reference Internal Under Voltage Lockout protects MOSFET during start-up

APPLICATIONSAPPLICATIONS

Single input Switching Regulators such as Simple 5V to 2.5V switcher for RDRAM regulator

US3036

8 PIN PWM SWITCHER

CONTROLLER IC

PRELIMINARY DATASHEET

DESCRIPTIONDESCRIPTION

The US3036 IC provides an 8 pin low cost switching controller with true short circuit protection all in a compact 8 pin surface mount package, providing a low cost switching solution for applications that re-

quire a simple switching regulator from the 5V input where there is no other supply available. One type of such application is generating 2.5V standby from the dual 5V(5V and 5V standby) for the next generation processors. The IC starts below 4.5V supply and drives an external Pch MOSFET or and external low cost PNP as the switching element. The IC also includes an error comparator for fast transient response, a precise voltage reference for setting the output voltage as well as a direct drive of the MOSFET for the minimum part count.

TYPICAL APPLICATIONTYPICAL APPLICATION

ATX P.S.

5V(S.B.)

5678

GndCS-CS+SS

US3036

Drv Vcc Vfb NC

1 2 3 4

R3 C5

D1R2

Typical application of US3036

Vout

3036app2-1.0

PACKAGE ORDER INFORMATIONPACKAGE ORDER INFORMATION

TA (°C) 8 PIN PLASTIC SOIC (S) 0 TO 70 US3036CS

Rev. 1.1 12/4/98

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US3036

ABSOLUTE MAXIMUM RATINGSABSOLUTE MAXIMUM RATINGS

VCC Supply Voltage ............................................................. 20V

F.B Pin Voltage........................................................ -0.3V to 5V

Storage Temperature Range ................................. -65 TO 150°C

Operating Junction Temperature ............................... 0 TO 150°C

PACKAGE INFORMATIONPACKAGE INFORMATION

8 PIN PLASTIC SOIC (S)

TOP VIEW

Drv

Vcc

Vfb

NC Gnd

θJA =160°C/W

CS+

CS-

ELECTRICAL SPECIFICATIONSELECTRICAL SPECIFICATIONS

Unless otherwise specified the following specification applies over VCC =5V, and TA =0 to 70°C. Low duty cycle pulse testing are used which keeps junction and case temperatures equal to the ambient temperature.

PARAMETER SYM TEST CONDITION MIN TYP MAX UNITS

F.B Voltage Initial Accuracy VFB TJ =25°C 1.237 1.250 1.262 V F.B Voltage Total Variation 1.225 1.250 1.275 V F.B Voltage Line Regulation 0.2 % F.B Input Bias Current IFB VFB =1.25V -1 +1 uA Min On Time VFB is sq wave with 300 ns on 800 nS

time and 2 uS off time

Min Off Time VFB is sq wave with 300 ns off 800 nS

time and 2 uS on time Supply Current ICCSW VFB =1.5V 10 mA Maximum Duty Cycle DMAX VFB =1.5V 100 % Minimum Duty Cycle DMIN VFB =1V 0 % Gate Drive Rise/Fall Time VGATE Load=1000pF 70 nS C.L Threshold Current ICL C.S+ , C.S- from 1.3V to 3.7V 20 uA C.S Comp Common Mode VCS+ = VCS- 0 4.5 V Soft Start Current 10 uA UVLO Threshold VUVLO 4.25 4.4 4.55 V

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PIN DESCRIPTIONSPIN DESCRIPTIONS

PIN # PIN SYMBOL PIN DESCRIPTION

3 VFB 6 C.S-

7 C.S+

5 Gnd 1 Drv

2 VCC

8 S.S

4 N.C

A resistor divider from this pin to the output of the switching regulator and ground sets the Core supply voltage. This pin is connected to the minus side of the external current sense resistor. An internal current source together with an external resistor in series with this pin programs the current limit threshold voltage. This voltage divided by the external current sense resistor sets the current limit threshold. This pin is connected to the plus side of the external current sense resistor. A resistor in series with this pin and a capacitor connected between this pin and pin 6 provides a high frequency filtering for the noise spikes of turn on and turn off switching. This pin is connected to the IC substrate and must be connected to the lowest potential in the system. The PWM output of the switching controller. This pin is a totem pole drive that is connected to the gate of the power MOSFET. A resistor may be placed from this pin to the gate in order to reduce switching noise. This pin supplies the voltage to the PWM drive and hysterises circuitry and it is connected to the same supply as the input supply to the switching regulator . A 1 uF, high frequency capacitor must be connected from this pin to ground to provide the peak current for charging and discharging of the MOSFET. This pin provides the soft start for the regulator during power up. It also sets a long off time when the converter goes into current limiting, providing low duty cycle for the catch diode allowing it to survive during short circuit. No connect.

US3036

BLOCK DIAGRAMBLOCK DIAGRAM

Vcc NC

2 4 3

Drv

Gnd

Vfb

5V Reg

UVLO

PWM Control

S.S. / Hiccup

Control

1.25V

20uA

Vref

3036blk1-1.2

Figure 1 - Simplified block diagram of the US3036

CS-

CS+

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US3036

TYPICAL APPLICATIONTYPICAL APPLICATION

5V to 2.5V for RDRAM Supply

5678

GndCS-CS+SS

US3036

Drv Vcc Vfb NC

1 2 3 4

R3 C5

ATX P.S.

5V(S.B.)

Figure 2- The circuit in figure 2 is the application of the US3036 providing a low cost solution for a 2.5V/2A supply from

the 5V dual supply. This circuit uses current sense resistor to set the current limiting.

D1R2

3036app1-1.3

Ref Desig Description Qty Part # Manufacturer

U1 LDO/Switcher IC 1 US3036CS ( 8 pin SOIC) Unisem Q2 MOSFET 1 IRF7204 (8 pin SOIC) or IRF7406 for 4A IR

P Ch SI9435DY (8 pin SOIC) or SI4431 for 4A Temic

D1 Schottky Diode 1 SK33DICT (SMC) Lite on

Vout

L2 Inductor 1 Core:T44-52,L=6 uH Micro Metal

Turns: 15T, 20 AWG (core) L1 Inductor 1 L=1 uH R1 Resistor 1 10 ohm,5%, SMT 1206 size R2 Resistor 1 10 ohm, 5%, SMT 1206 size R3 Resistor 1 200 kohm,1%, SMT 0805 size R4 Resistor 1 1 kohm,1%, SMT 0805 size R5 Resistor 1 1 kohm,1%, SMT 0805 size R6 Resistor 1 5 miliohm,5%, 2W Ohmite R7,8 Resistor 2 3.57 kohm,1%, SMT 0805 size C1 Capacitor 1 470uF,10V, Elect Sanyo C2 Capacitor 1 10CV1000DX, 1000uF,10V, Elect ,ESR=0.07 Ohm Sanyo C3 Capacitor 1 1 uF,Ceramic, SMT 0805 size C4 Capacitor 1 470 pF,Ceramic, SMT 0805 size Sanyo C5 Capacitor 1 10 pF,Ceramic, SMT 0805 size Sanyo C6 Capacitor 1 4700pF C7 Capacitor 1 10CV1000DX, 1000uF,10V, Elect ,ESR=0.07 Ohm Sanyo C8 Capacitor 1 0.15 uF

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TYPICAL APPLICATIONTYPICAL APPLICATION

5V to 2.5V for RDRAM Supply

US3036

5678

GndCS-CS+SS

US3036

Drv Vcc Vfb NC

1 2 3 4

R3 C5

ATX P.S.

5V(S.B.)

Figure 2- The circuit in figure 2 is the application of the US3036 providing a low cost solution for a 2.5V supply from

the 5V dual supply.

D1R2

3036app2-1.0

Ref Desig Description Qty Part # Manufacturer

U1 LDO/Switcher IC 1 US3036CS ( 8 pin SOIC) Unisem Q2 MOSFET 1 IRF7204 (8 pin SOIC) or IRF7406 for 4A IR

P Ch SI9435DY (8 pin SOIC) or SI4431 for 4A Temic

D1 Schottky Diode 1 SK33DICT (SMC) Lite on

Vout

L2 Inductor 1 Core:T44-52,L=6 uH Micro Metal

Turns: 15T, 20 AWG (core) L1 Inductor 1 L=1 uH R1 Resistor 1 10 ohm,5%, SMT 1206 size R2 Resistor 1 10 ohm, 5%, SMT 1206 size R3 Resistor 1 200 kohm,1%, SMT 0805 size R4 Resistor 1 1 kohm,1%, SMT 0805 size R5 Resistor 1 1 kohm,1%, SMT 0805 size R6 Resistor 1 1 kohm,5% R7 Resistor 1 3.57 kohm,1%, SMT 0805 size C1 Capacitor 1 470uF,10V, Elect Sanyo C2 Capacitor 1 10CV1000DX, 1000uF,10V, Elect ,ESR=0.07 Ohm Sanyo C3 Capacitor 1 1 uF,Ceramic, SMT 0805 size C4 Capacitor 1 470 pF,Ceramic, SMT 0805 size Sanyo C5 Capacitor 1 10 pF,Ceramic, SMT 0805 size Sanyo C6 Capacitor 1 0.1uF C7 Capacitor 1 10CV1000DX, 1000uF,10V, Elect ,ESR=0.07 Ohm Sanyo C8 Capacitor 1 0.15 uF

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US3036

APPLICATION INFORMATIONAPPLICATION INFORMATION

Introduction

The US3036 device is an application specific product designed to provide an on board switching supply for the new generation of microprocessors requiring separate Core and I/O supplies where the load current demand from the I/O supply requires this regulator to also be a switching regulator such as the motherboard applications with AGP slot or the Pentium II with on board 5V to

3.3V converter. The US3036 provides an easy and low cost switching regulator solution for Vcore and 3.3V supplies with true short circuit protection.

Switching Controller Operation

The operation of the switching controller is as follows : after the power is applied, the output drive pin, "Drv" goes low turning the P MOS to 100% duty cycle and the the current in the inductor charges the output capacitor causing the output voltage to increase. When output reaches a pre-programmed set point the feedback pin "Vfb" exceeds 1.25V causing the output drive to switch low and the "Vhyst" pin to switch high which jumps the feedback pin higher than 1.25V resulting in a fixed output ripple which is given by the following equation : dVo=(Rt/Rh)x(Vcc -1) Where: Rt=Resistor connected from Vout to the Vfb pin of US3036 Rh=Resistor connected from Vfb pin to Vhyst pin. For example, if Rt=1k and Rh=200k, then the output ripple is : dVo=(1/200)x4=20 mV The advantage of fixed output ripple method is that when the output voltage changes from 2V to 3.5V, the ripple voltage remains the same which is important in meeting the Intel maximum tolerance specification.

Soft Start

The soft start capacitor must be selected such that during the start up when the output capacitors are charging up, the peak inductor current does not reach the current limit treshold. A minimum of 0.1uF capacitor insures this for most applications. During start up the soft start capacitor is charged up to approximately 6V keeping the output shutdown before an internal 10uA current source start discharging the soft start capacitor which slowly ramps up the inverting input of the PWM comparator, Vfb. This insures the output to ramp up at the same rate as the soft start cap thereby limiting the input current. For example, with 0.1uF and the 10uA internal current source the ramp up rate is (∆V/ ∆t)=I/Css = 10/

0.1=100V/Sec or 0.1V/mSec. Assuming that the output capacitance is 6000uF, the peak input current will be: Iin(pk)=Css*(∆V/ ∆t)=6000uF*(0.1V/mSec)=0.6A

The soft start capacitor also provides a delay in the turn on of the output which is given by: Td=CSS*K Where K=30 ms/uF For example for CSS=0.1uF, Td=0.1* 30=3 ms

Switcher Current Limit Protection

The US3034 uses an external current sensing resistor and compares the voltage drop across it to a programmed voltage which is set externally via a resistor (RcL) placed between the “CS-” terminal of the IC and Vout. Once the voltage across the sense resistor exceeds the threshold, the soft start capacitor pulls up to 12V, pulling up the inverting pin of the error comparator higher than non inverting which causes the external MOSFET to shut off. At this point the C.S comparator changes its state and pulls the soft start capacitor to Vcc which is 12V and shutting the PWM drive. After the output drive is turned off, an internal 10uA current source slowly discharge the soft start capacitor to approximately 5.7V, before the output starts to turn back on causing a long delay before the MOSFET turns back on. This delay causes the catch diode to cool off between the current limit cycles allowing the converter to survive a short circuit condition. An example is given below as how to select the current limiting components. Assuming the desired current limit point is set to be 20A and the current sense resistor Rs=5mΩ, then the current limit pro- gramming resistor,RcL is calculated as : Vcs=IcL*Rs=20*0.005=0.1V RcL=Vcs/Ib=(0.1V)/(20uA)=5kΩ Where: Ib=20uA is the internal current source of the US3034 The peak power dissipated in the C.S. resistor is : Ppk=(IcL^2)*Rs=20^2*0.005=2W However, the average power dissipated is much lower than 2W due to the long off time caused by the hiccup circuit of 3034. The average power is in fact the short circuit period divided by the short circuit period plus the off time or "hiccup" period. For example, if the short circuit lasts for TSC=100uSec before the 3034 enters hiccup, the average power is calculated as : Pave=Ppk*DSC Where: DSC=TSC/THCP THCP=CSS*M Where M=630 ms/uF & CSS, is the soft start capacitor For example for CSS=0.1uF & TSC=100uSec=0.1mS THCP=0.1* 630=63 ms Pave=2*(0.1/63)=3.2 mW

Without "hiccup" technique, the power dissipation of the resistor is 2W.

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Switcher Output Voltage Setting

The output voltage can be set using the following equations. Assuming , Vo=3.38V and the selected output ripple is ≈ 1.3%(44mV) of the output voltage, a set of equations are derived that selects the resistor divider and the hysterises resistor. Assuming, Rt=1kΩ , 1% Rh=(11*Rt)/∆Vo Where: Rt=Top resistor of the resistor divider Rh=Hysterises resistor connected between pins 3 and 4 of the US3034 ∆Vo=Selected output ripple (typically 1% to 2% of output voltage) Assuming, ∆Vo=44mV Rh=(11*1000)/0.044=250 kΩ Select Rh=249 kΩ , 1% The bottom resistor of the divider is then calculated using the following equations: Rb=Rt/X Where: Rb=Bottom resistor of the divider X=[(Vo + (∆Vo/2))/Vref] - 1 Vref=1.25 V typ. X=[(3.38+ (0.044/2))/1.25] - 1 = 1.72 Rb=1000/1.72=580 Ω Select Rb=576 Ω , 1%

US3036

Frequency Calculation

The US3034 frequency of operation is calculated using the following formula: Fs=[(Vo*(1-D)*ESR)]/(L*∆Vo) (MHz) Where: Vo=Output voltage (V) D=Duty cycle ESR=Output capacitor ESR (V) L=Output inductance (uH) ∆Vo=Output ripple voltage (V) For our example: D≈(Vo + Vf)/Vin Where, Vf=Forward voltage drop of the Schotky diode D=(3.38 + 0.5)/5=0.78 The ESR=18mΩ for 2 of the Sanyo 1500uF, 6MV1500GX caps. If L=3.5uH then, Fs is calculated as follows: Fs=[(3.38*(1-0.78)*0.018)]/(3.5*0.044)= 0.087 Mhz = 87 kHz

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