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SP7606
Wide Input Voltage Boost Controller
FEATURES * Fixed Frequency 1200kHz Voltage-Mode PWM Operation * Requires Tiny Inductors and Capacitors * Adjustable Output Voltage up to 38V * Up to 85% Efficiency * Internal Compensation * Built in current limit * Low Supply Current * 8-pin 2x3 DFN PINOUT
DD OC GND GATE 1 2 3 4 8 VCC SHDN VIN FB
SP7606
2x3 8 Pin DFN
7 6 5
Now Available in Lead Free Packaging
APPLICATIONS * White LED Backlighting when combined with SP7615 or SP7616 * Large LED arrays for general lighting * General boost, flyback, or SEPIC converters GENERAL DESCRIPTION
The SP7606 is a fixed frequency boost controller designed to drive loads up to 38V output voltage. The SP7606 was developed to be used in conjunction with the SP7616 to drive a wide range of led chains that require high anode voltages. The ability to disconnect the output voltage feedback resistors (DD Pin) reduces shutdown current. The high switching frequency allows the use of tiny external components and saves layout space and cost. The SP7606 is available in a space-saving 8-pin 2x3 DFN.
TYPICAL APPLICATION SCHEMATIC Boost converter 12V to 30V ABSOLUTE MAXIMUM RATINGS
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SP7606
These are stress ratings only and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifications below is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. Vin....................................................-0.3V to 30V EN, FB, SHDN, and Vcc ....................-0.3V to 6V DD, OC..............................................0.3V to 40V Storage Temperature................ -65C to +150C Lead Temperature (Soldering, 10 sec) ......300C
RECOMMENDED OPERATING CONDITIONS
Supply Voltage (Vin)............................ 7V to 28V Operating temperature ................. -40TJ125C ESD Rating SHDN pin...................... 1.5KV HBM ESD Rating all other pins .................... 2KV HBM Package Thermal Dissipation..................45C/W
ELECTRICAL CHARACTERISTICS
Specifications are for TAMB=TJ=25C, and those denoted by apply over the full operating range, -40C TJ 125C Unless otherwise specified: Vin = 7 -28V, CGATE = 1000pF,
PARAMETER Operating Input Voltage Range Supply Current Supply current in shutdown Vcc Output Voltage Vcc Dropout Voltage Under Voltage Lockout Switching Frequency Maximum Duty Cycle Minimum On-time Turn-on Time from Shutdown FB reference Voltage FB Input Current Error amplifier gain** Ramp Amplitude** Gate Rising Time Gate Falling Time Gate Pull-up Resistance Gate Pull-down Resistance Gate Pull Down Resistance in off state SHDN Logic Low SHDN Logic High SHDN Input Current Over-Current Protection threshold Over-Current Trip Point Delay DD FET impedance
MIN 7
TYP
MAX 28
UNITS V mA A V V V MHz % ns s mV A dB V ns k V V A V ns

CONDITIONS Not switching Vin=28V SHDN_ = HIGH Icc = 20mA
2.2 55 4.8 4.1 1.0 86 5.0 4.35 1.20 30 200 800 80 Vin/10
4.2 160 5.3 2 4.6 1.4 91 500 816 0.5
0CTJ85C VFB = 1V
784
1.25
4.2 50 40
10 to 90% 90 to 10%
4 3 50 0 3 .01 0.145 0.20 155 .7 5.3 0.5 0.260 100

Enabled Disabled 0 to Vcc
*Not tested but the specification is guaranteed by design.
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SP7606
PIN ASSIGNMENTS Pin Name DD OC GND GATE Vcc SHDN Vin FB Pin# 1 2 3 4 5 6 7 8 Pin Description Divider disconnect; Upper resistor of output voltage setting divider is connected to this point Over -current protection Ground pin Gate pin. Connect external MOSFET gate to this pin. Minimize trace area to reduce EMI Internal circuit power source. Bypass Vcc to GND with 0.1F capacitor. Shutdown pin. Device is active if SHDN is logic LOW (<0.7V) Power input pin. Bypass Vin to GND with 1F capacitor as close to Vin as possible Feedback pin. Reference voltage is 0.8V
BLOCK DIAGRAM
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SP7606
Typical Performance Characteristics
1.40 1.35 1.30 Frequency MHz 1.25 1.20 1.15 1.10 1.05 1.00 -40 -30 -20 -10 0 10
SP7606
25
35
55
75
85
95
105
115
125
Temperature C
Oscillator Frequency vs Temperature
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SP7606
CIRCUIT DESCRIPTION THEORY OF OPERATION The SP7606 converter is a voltage mode boost controller. The control loop has built in Type 2 compensation coupled with high switching frequency allows the user to use small components when designing the output filter. The equations below show generic relationships as applicable to the boost regulator running in discontinuous conduction mode (DCM) and continuous conduction mode (CCM) of operation. Duty Cycle in continuous conduction mode (CCM) The output voltage of the SP7606 can be set by using an output voltage divider. The internal reference of this part is set to 0.8V. Due to the internal compensation, resistor R1 might need to be chosen according to the desired gain of the compensation loop. This resistor is typically between 100K and 1M ohm. Resistor R2 can be determined by:
Where
Vd= Forward voltage drop of D1
Vfb=800mV Feedback Voltage R1=Top Voltage divider resistor R2=Bottom Voltage divide resistor 200=the typical impedance of the DD FET
Duty Cycle in discontinuous conduction mode (DCM)
For typical applications resistor R1 should be connected between Vout and the DD pin. The DD pin serves as a disconnect for the output voltage divider when the SP7606 is disabled. This feature allows the user to save power when the converter is not running. The typical Impedance of the DD FET when enabled is 200 ohms. If the DD pin is not used the resistor R1 can be connected directly to the Vfb pin to get the proper output voltage. If this type of connection is used the Rdson of the DD Fet can be ignored and the equation becomes:
Where fsw is the switching frequency L is the inductor Re is the effective resistance of the small signal model
Vfb=800mV Feedback Voltage R1=Top Voltage divider resistor R2=Bottom Voltage divide resistor
Re can be found by using as follows:
Setting the output voltage
If the DD pin is not used the converter can be used to boost voltages beyond 38V. A more detailed discussion on this topic can be found in the section "High Voltages Operation". A 10 to 22pf decoupling capacitor from the feedback pin to ground is recommended when the SP7606 is used with resistor values above 20K in the voltage divider circuit.
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SP7606
Over current protection The boost regulator topology inherently does not have short circuit protection. The SP7606 converter uses a simple comparator circuit to check for an over current condition on a pulse by pulse basis. The Vset voltage threshold for the over-current (OC) pin is set to 0.25V. Current limit set point is: Current sense using resistor The approximate associated power loss in the resistor is:
Rsense=Current sense resistor
Rdson current sense Typically the converter current limit is set to about 150% of the normal output current. This allows the converter to function at maximum output current without accidentally triggering the current limit. 150% over current limit takes into effect the variations in RDSon of the FET, as well as the inductor inductance values. The accuracy of the current sensing can be increased by the use of a sense resistor. The resistor values tend to be more accurate down to 1%.
For continuous conduction mode the ton/T becomes the duty cycle of the converter. For DCM mode the value of K can be used. The other benefit of this combination is that the OC pin does not see the high voltages and the converter can be used to generate much higher voltages then 38V. Please refer to the high output voltage operation section for a more detailed explanation. The over current protection can be disabled by tying the OC pin to GND. High Voltage Operation The converter can be used to boost voltage to higher than the rated voltage of the DD pin and the OC pin. To do this two things need to be done for proper operation. 1) The voltage set resistors need to be connected directly to the Vfb pin bypassing the DD internal FET. By doing this the user can get voltages that are higher than Vout of 38V. By doing this the user does not have the output voltage disconnect feature. 2) The second thing that needs to be done is the circuit needs to use a current sense resistor for current limit. This prevents the OC pin from seeing high
page 6 of 17
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SP7606
switch-node voltages. The current sense resistor schematic setup is shown in Figure 2.
Vo is the output voltage Vinmin is the minimum input voltage
3 The inductor inductance is:
Schematic for High Voltage Operation Other Topologies The SP7606 is not only capable of driving boost regulator circuits, but can also be used in flyback and SEPIC topologies. Look for an application note in the future on EXAR's web site Inductor selection Typically the inductor needs to be chosen for its current capability and size. For most applications using the SP7606 the inductor should be chosen so that at light loads the inductor runs in discontinuous mode and then enters continuous mode of operation at high loads. This allows the inductor to be reasonably sized and helps with compensation of the overall circuit as well. The procedure for selecting the inductor current for discontinuous mode of operation is as follows. 1 The first thing that needs to be determined is the constant K. K represents a ratio of MOSFET conduction to diode conduction. Typically a value of 0.8 can be used, this will assure that there is about 20% dead time present to have DCM mode of operation. 2 The on time is calculated is:
Where K is .8 ratio of MOSFET and diode conduction time to T (T=1/fsw) = output impedance at full load. Vo is the output voltage Vinmin is the minimum input voltage Ton is the maximum on time
4 The inductor peak current Ip is:
Although the SP7606 typical mode of operation is in DCM mode, due to easier compensation below are the formulas for when the SP7606 runs in CCM. For continuous conduction mode the inductor current is:
fsw is the switching frequency Vd is forward diode drop of D1 Vo is the output voltage Vinmin is the minimum input voltage
The approximate peak inductor current is:
MOSFET selection
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SP7606
The MOSFET needs to be chosen based on three main criteria. The drain to source voltage needs to be higher than the output voltage of the converter. The MOSFET needs to be able to conduct the peak current that is calculated in the inductor selection section. The Rdson of the MOSFET needs to satisfy current limit criteria. Picking a MOSFET with the lowest Qg and Crss that meets the above requirement is crucial to good efficiency. At 1.2MHz the switching losses become a significant power loss in the system even compared to the Rdson of the MOSFET. For continuous conduction mode the power loss in the MOSFET is:
The gate drive loss is associated with the FET but it actually is lost in the driver IC. Below is the formula for the gate charge loss Qg.
5V is the gate drive voltage Qg is the total gate charge Fsw is the switching frequency
Where In max is the maximum input current D is the duty cycle Vo is the output voltage Crss reverse transfer capacitance of MOSFET fsw is the switching frequency kt is the temperature dependency of RDSon kg is the constant inversely proportional to gate drive current a value of 1.5 should be used.
Input capacitor selection For both continuous and discontinuous mode of operation the input capacitor needs to be chosen based on maximum input voltage rating and the RMS ripple current and minimum input capacitance. For DCM mode the RMS current is given by:
Where K is the conduction time constant Ip is the peak inductor current
For discontinuous mode of operation the power loss in the FET will is similar.
The minimum required is:
input
capacitance
that
is
Ton is the calculated on time Vin is the minimum input voltage
Crss reverse transfer capacitance of MOSFET Fsw is the switching frequency is the temperature dependency of RDSon Z is the constant inversely proportional to gate drive current Where In max is the maximum input current K is the duty cycle Vo is the output voltage
For CCM of operation the input capacitor ripple current is:
Where Iinrms
Fsw is the switching frequency Vo is the output voltage Vinmin is the minimum input voltage L inductor inductance
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SP7606
Output capacitor selection For best performance a combination of both electrolytic and ceramic capacitors should be used. For both DCM and CCM mode the required ESR is approximately given by
Gain (dB)
Frequency (Hz)
For CCM mode of operation the output capacitor ripple is approximately:
Bode plot of type two compensation The internal pole and zero location for the SP7606 internal compensation is
The minimum output capacitance required in CCM and DCM mode is approximated by
Zero Location= 5.3KHz Pole Location=398KHz Modulator Gain CCM (feed forward) The SP7606 has also built in a feed forward topology to allows the boost regulator to have the same modulator gain throughout its full input voltage range swing when running in CCM. The modulator gain is for the SP7606 is
Where D= duty cycle for different modes of operation fsw=switching frequency Error Amplifier The SP7606 has built in internal Type 2 compensation.
RZ CP Vout Comp R1
+
CZ
Modulator Gain DCM (feed forward) The SP7606 has also built in a feed forward topology to allow the boost regulator to have the same modulator gain throughout its full input voltage range swing when running in DCM. The modulator gain is for the SP7606 is
Vref
Type II compensation The values for Rz is 200K Cz is 150pF CP is 2pF R1 is chosen for proper gain.
Where M= D= Duty Cycle in DCM
Boost regulator output filter DCM When a boost regulator is running in discontinuous conduction mode the output
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SP7606
filter characteristics are composed of a single pole and a single zero. The following equations show the location of the pole and zero for the output filter of the boost regulator.
ESRzero =
1 2 Cout Cesr
Gain dB
0
0 deg Phase deg -90 (deg) Frequency Hz
Gain of Control to Output transfer DCM
For most applications that operate in discontinues conduction mode the internal compensation is sufficient and no external compensation is required. Boost regulator output filter CCM When a boost regulator is running in continuous conduction mode the output filter characteristics are composed of a filter double pole an ESR_zero and a right half plane (RHP) zero.
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SP7606
Gain
0 dB
0 deg Phase -270 (deg) Frequency
Gain of Control to Output transfer CCM
Vout
The compensation becomes much harder to accomplish when due to the RHP zero as well as the filter double pole. Unlike the DCM filter the gain drops off sharply at the filter double pole and does not recover. More over the Right Half plane zero also adds another -90 degrees to the phase. Due to the RHP zero the compensation network for a boost regulator needs to roll off below the RHP zero location. When compensating for CCM mode of operation the user will need to add a phase boost capacitor and resistor to help compensate for the filter double pole. The location of the zero and poles for Type 3 compensation is.
RZ CP
CZ CZ1 RZ1
Comp
7
+
5 6
R1
Vref
Error Amplifier Type 3 Compensation
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SP7606
Gain
0 dB
Type 3 Error amplifier Gain Plot
The compensation needs to be such that the Z2 phase boost zero needs to be located around the filter double pole to help offset the filter double pole. But the overall crossover frequency needs to occur below the RHP zero. BOARD LAYOUT AND GROUNDING To obtain the best performance from the SP7606, a printed circuit board with ground plane is required. High quality, low series resistance ceramic bypass capacitors should be used at the Vin and Vout pins (pins 1 and 8). These capacitors must be located as close to the pins as possible. The traces connecting the pins and these capacitors must be kept short and should be made as wide as possible. Below is a Typical Layout for the SP7606.
Frequency
Top Side
Bottom Side
SP7606 WAVEFORMS
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SP7606
400mA Load switching characteristics 12Vin 30Vout Channel 1 Vout Ripple Channel 2 LX node Channel 3 Inductor Current 2A/Div
Light Load switching characteristics 12Vin 30Vout Channel 1 Vout Ripple Channel 2 LX node Channel 3 Inductor Current 2A/Div
Transient Response load step 100mA to 400mA
Startup characteristics into 400mA load Channel 1 Vout Channel 2 Vin
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SP7606
Efficiency Graph 12Vin 30Vout TYPICAL APPLICATIONS
Boost converter with Over Voltage/Over current Protection for LED driving
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SP7606
12 VIN 2.2 H MBR160 50 VOUT
CIN 10 F 6 VIN 8 VCC 7 SHDN 3 GND 1F 3.2 k GATE FB 10 pF 4 OC 5 DD 2 1
200 k
10 F COUT
High output voltage solution
5 VIN
2.2 H
MBR160
VOUT
CIN 10 F
200 k
10 F COUT
6 VIN 8 7 3 GND VCC GND PAD SHDN DD OC FB GATE
1 2 5 10 pF 4
1F 5.4 k
5Vin to 30Vout
8-PIN 2 x 3 mm DFN PACKAGE DIMENSIONS
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SP7606
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SP7606
ORDERING INFORMATION Part Number Junction Temperature Range Package SP7606ER-L ................. -40oC to +125oC....................................Lead Free 8-PIN 2 x 3 mm DFN SP7606ER-L/TR............ -40oC to +125oC........... Tape and Real Lead Free 8-PIN 2 x 3 mm DFN Pack Quantity for tape and real is 3000 REVISION HISTORY DATE December 2007 REVISION A DESCRIPTION Original Release
For further assistance: Email: EXAR Technical Documentation: customersupport@exar.com http://www.exar.com/TechDoc/default.aspx?
Exar Corporation Headquarters and Sales Office 48720 Kato Road Fremont, CA 94538 main: 510-668-7000 fax: 510-668-7030
EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a user's specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances.
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