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Preliminary
RT9277A/B
High Performance, Low Noise Boost Converter
General Description
The RT9277A/B is a high performance, low noise, fixed frequency step up DC-DC Converter. The RT9277A/B converters input voltage ranging 2.5V to 5.5V into output voltage up to 16V. Current mode control with external compensation network makes it easy to stabilize the system and keep maximum flexibility. Soft start function minimizes impact on the input power system. Internal power MOSFET with very low RDS(ON) provides high efficiency. The RT9277A/B automatically transits from PWM to PSM (Pulse Skipping Mode) during light load condition further increasing efficiency. 640kHz and 1.2MHz operation frequency options provide flexibiltity of minimum output inductor size, maximum efficiency and low BOM cost. The RT9277A/B also provides comprehensive protection functions such as UVLO, OCP and OTP.
Features
90% Efficiency VIN Operating Range : 2.5V to 5.5V 1.6A, 0.2, 16V Internal Power MOSFET 640kHz and 1.2MHz Operation Frequency External Compensation Network Internal/External Programmable Soft Start Function Small MSOP8 Package OCP and OTP Function are Included RoHS Compliant and 100% Lead (Pb)-Free
Applications
TFT LCD panel OLED Display PCMCIA Cards Portable Device
Ordering Information
RT9277A/B Package Type F : MSOP-8 Operating Temperature Range P : Pb Free with Commercial Standard G : Green (Halogen Free with Commercial Standard) Soft Start Function A : Internal B : External Programmable
Pin Configurations
(TOP VIEW)
COMP FB EN GND 1 2 3 4 8 7 6 5 NC FREQ VIN LX
RT9277A
Note : RichTek Pb-free and Green products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. 100%matte tin (Sn) plating.
COMP FB EN GND
1 2 3 4
8 7 6 5
SS FREQ VIN LX
RT9277B MSOP-8
Marking Information
For marking information, contact our sales representative directly or through a RichTek distributor located in your area, otherwise visit our website for detail.
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RT9277A/B
Typical Application Circuit
Preliminary
Chip Enable VIN 2.5V to 5.5V C1 L1 + 5 LX 6 VIN GND 4 EN 3 R3 C4 RT9277A C3 R2 7 FREQ FB 2 8 NC COMP 1 D1 VOUT C2 R1 +
Figure 1
Chip Enable VIN 2.5V to 5.5V C1 L1 + 5 LX 6 VIN GND 4 EN 3 R3 C4 RT9277B C3 R2 7 FREQ FB 2 8 SS COMP 1 CSS D1 VOUT C2 R1 +
Figure 2 Table 1. Component Selection FOSC C1 L1 C2 (Hz) (uF) (uH) (uF) 1.2M 10 4.7(TDK SLF6028) 33 (ceramic)
Symbol (unit) Application 1 Application 2 Application 3
VIN (V) 3.3 3.3 3.3
VOUT (V) 9 12 12
R3 (k) 82 180 120
C3 (pF) 820 680 1200
C4 (pF) 10 22 22
1.2M 640K
10 10
4.7(TDK SLF6028) 10(TDK SLF6028)
33 (ceramic) 33 (ceramic)
Function Block Diagram
LX VIN VFB Error Amplifier + Protection Internal Soft-Start
EN COMP
VIN
FB 1.24V
Summing Comparator + Clock
Control and Driver Logic
LX N GND
5uA FREQ Oscillator Slope Compensation Current Sense
RT9277A
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Preliminary
RT9277A/B
4uA VIN
EN COMP
LX VIN VFB Error Amplifier +
Protection SoftStart Summing Comparator + Clock SS LX N GND
FB 1.24V
Control and Driver Logic
5uA FREQ Oscillator Slope Compensation Current Sense
RT9277B
Operation
The RT9277A/B is a high efficiency step-up Boost converter with a fixed-frequency, current-mode PWM architecture. It performs fast transient response and low noise operation with appropriate component selection. The output voltage is regulated through a feedback control consisting of an error amplifier, a summing comparator, and several control signal generators (as shown in function block diagram). The feedback reference voltage is 1.24V. The error amplifier varies the COMP voltage by sensing the FB pin. The slope compensation signal summed with the current -sense signal will be compared with the COMP voltage through the summing comparator to determine the current trip point and duty cycle. When driving light loads, the RT9277A/B will perform the pulse-skipping mode to prevent overcharging the output voltage. In this mode, the switching frequency will be reduced to perform a higher efficiency. Soft-Start The RT9277B provides programmable soft-start function. When the EN pin is connected to high, a 4A constant current is sourced to charge an external capacitor. The voltage rate of rise on the COMP pin is limited during the charging period, and so is the peak inductor current. When the EN pin is connected to GND, the external capacitor will be discharged to ground for the next time soft-start. Current Limitation The switch current is monitored to limit the value not to exceed 1.6A typically. When the switch current reaches 1.6A, the output voltage will be pulled down to limit the total output power to protect the power switch and external components. Shutdown Connect the EN to GND to turn the RT9277A/B off and reduce the supply current to 0.1A. In this operation, the output voltage is the value of VIN to subtract the forward voltage of catch diode. Frequency Selection The switching frequency of RT9277A/B can be selected to operate at either 640kHz or 1.2MHz. When the FREQ pin is connected to GND for 640kHz operation, and connected to VIN for 1.2MHz operation. FREQ is preset to 640kHz operation for allowing the FREQ pin unconnected.
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RT9277A/B
Functional Pin Description
Pin No. RT9277A 1 2 3 4 5 6 7 RT9277B 1 2 3 4 5 6 7 Pin Name
Preliminary
Pin Function Compensation Pin for Error Amplifier. Connect a compensation network to ground. See the Component Selection Table for the loop compensation. Feedback Pin. Connect an external resistor-divider tap to FB. The typical reference voltage is 1.24V. Shutdown Control Input. Connect EN to GND to turn off the RT9277A/B. Ground Switch Pin. Connect the inductor and catch diode to LX pin. Widen and shorten the connected trace to minimize EMI. Supply Pin. Place at least a 1F ceramic capacitor close to RT9277A/B for bypassing noise. Frequency Select Pin. Oscillator frequency is 640kHz as FREQ connected to GND, and 1.2MHz as FREQ connected to VIN. A 5A pull-down current is sinking on this pin. Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. A 4A constant current charges the soft-start capacitor. When EN connected to GND, the soft-start capacitor is discharged. When EN connected to VIN high, the soft-start capacitor is charged to VIN. Leave floating for not using soft-start. No Connection
COMP FB EN GND LX VIN FREQ
--
8
SS
8
--
NC
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Preliminary Absolute Maximum Ratings
(Note 1)
RT9277A/B
Supply Voltage (VIN) -------------------------------------------------------------------------------------------------- -0.3 to 6V LX to GND --------------------------------------------------------------------------------------------------------------- - 0.3V to 16V The other pins ---------------------------------------------------------------------------------------------------------- - 0.3V to 6V Power Dissipation, PD @ TA = 70C MSOP-8 ----------------------------------------------------------------------------------------------------------------- 625mW Package Thermal Resistance (Note 4) MSOP-8, JA ------------------------------------------------------------------------------------------------------------ 160C/W Junction Temperature ------------------------------------------------------------------------------------------------- 150C Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260C Storage Temperature Range ---------------------------------------------------------------------------------------- - 65C to 150C ESD Susceptibility (Note 2) HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 3)
Junction Temperature Range ---------------------------------------------------------------------------------------- -40C to 125C Ambient Temperature Range ---------------------------------------------------------------------------------------- -40C to 85C
Electrical Characteristics
(VIN = 3V, FREQ left floating, TA = 25C, Unless Otherwise specification)
Parameter System Supply Input Operation voltage Range Under Voltage Lock Out Power On Reset Hysteresis Quiescent Current Shut Down Current Soft start Current (RT9277B) Switching Regulator Oscillator Free Run Frequency Maximum Duty Cycle Reference Voltage Feedback Reference Voltage Error Amplifier Transconductance Voltage Gain Gm AV 70 -140 700 240 - V/V VREF VCOMP = 1.24V 1.222 1.24 1.258 V FREQ = GND fOSC FREQ = VIN 540 -82 640 1200 90 740 -96 kHz kHz % IQ ISHDN ISS VFB = 1.3V, No switching VFB = 1.0V, Switching, No load EN = GND VSS = 1.2V VIN UVLO 2.5 1.9 ----1.5 -2 100 250 2 -4 5.5 2.1 -350 5 1 7 V V mV A mA A A Symbol Test Condition Min Typ Max Units
To be continued
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RT9277A/B
Parameter Feedback Voltage Line Regulation MOSFET On Resistance of MOSFET Current Limitation Enable Control Input Input Low Voltage Input High Voltage Hysteresis Protection Function Over Temperature Protection Hysteresis VIL VIH RDS(ON) Symbol
Preliminary
Test Condition VCOMP = 1.24V, 2.5V < VIN < 5.5V -1.2 2.5V VIN 5.5V 2.5V VIN 5.5V -0.7 x VIN ---200 1.6 --0.1 170 20 500 -m A V V V C C Min -Typ 0.05 Max 0.15 Units %/V
0.3 x VIN -----
Note 1.Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. Note 2. Devices are ESD sensitive. Handling precaution recommended. Note 3. The device is not guaranteed to function outside its operating conditions. Note 4. JA is measured in the natural convection at TA = 25C on a low effective single layer thermal conductivity test board of JEDEC 51-3 thermal measurement standard.
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Preliminary Typical Operating Characteristics
Efficiency vs. Output Current
95
100
RT9277A/B
Efficiency vs. Output Current
640kHz
640kHz
90 85
1.2MHz
90
1.2MHz
Efficiency (%)
Efficiency (%)
VIN = 3.3V, VOUT = 12V
80 75 70 65 60 55
80
70
60
VIN = 5.0V, VOUT = 12V
50 0 50 100 150 200 250 300 350 400
50 0 20 40 60 80 100 120 140 160 180 200
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
100
Output Voltage vs. Output Current
12.06
VIN = 5V
90
12.04
Efficiency (%)
VIN = 3.5V
80
Output Voltage (V)
12.02 12.00 11.98 11.96
640kHz
1.2MHz
70
60
VOUT = 9.0V, f = 1.2MHz
50 0 50 100 150 200 250 300 350 400
VIN = 3.3V, VOUT = 12V
11.94 0 20 40 60 80 100 120 140 160 180 200
Output Current (mA)
Output Current (mA)
Output Voltage vs. Output Current
12.06 12.04
Output Voltage vs. Output Current
9.00
8.96
Output Voltage (V)
12.00
Output Voltage (V)
12.02
1.2MHz
11.98 11.96 11.94 11.92
8.92
VIN = 3.5V VIN = 5V
640kHz
8.88
8.84
VIN = 5.0V, VOUT = 12V
11.90 0 50 100 150 200 250 300 350
VOUT = 9.0V, f = 1.2MHz
8.80 0 50 100 150 200 250 300 350
Output Current (mA)
Output Current (mA)
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RT9277A/B
Preliminary
Output Voltage vs. Input Voltage
11.986 11.984
Output Voltage vs. Input Voltage
11.998 11.996
640kHz
1.2MHz
Output Voltage (V)
11.982 11.980 11.978 11.976 11.974
Output Voltage (V)
11.994 11.992 11.990 11.988 11.986
VOUT = 12V, IOUT = 1mA
11.972 2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5 4.8 5.0
11.984 2.5 2.8 3.0 3.3 3.5 3.8
VOUT = 12V, IOUT = 1mA
4.0 4.3 4.5 4.8 5.0
Input Voltage (V)
Input Voltage (V)
No Load Supply Current vs. Input Voltage
500
Current Limit vs. Input Voltage
1.8 1.7
640kHz
640kHz
No Load Supply Current (uA)
450 400 350 300 250 200
Current Limit (mA)
VOUT = 12V
1.6 1.5 1.4 1.3
VOUT = 12V
1.2 2.5 3.0 3.5 4.0 4.5 5.0 5.5
150 2.5 3 3.5 4 4.5 5 5.5
Input Voltage (V)
Input Voltage (V)
VFB vs. Temperature
1.238
1300
Frequency vs. Temperature
1.2MHz 1.2MHz
1.238 1.237 1.237 1.236 1.236 1.235 1.235 1.234 1.234 -40 -20 0 20 40 60 80 100
1200
Frequency (kHz)
VIN = 3.3V, VOUT = 12V
V FB Voltage(V)
1100
1000
900
VIN = 3.3V, VOUT = 12V
800 -40 -20 0 20 40 60 80 100
Temperature (C)
Temperature (C)
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Preliminary
RT9277A/B
Power Off
Start Up
VL1 (10V/Div)
VL1 (10V/Div)
VOUT (5V/Div) VEN (5V/Div) ILOAD (1A/Div)
VIN = 3.3V, IOUT = 200mA, f = 640kHz
VOUT (5V/Div) VEN (5V/Div) ILOAD (1A/Div)
VIN = 3.3V, IOUT = 200mA, f = 640kHz
Time (2.5ms/Div)
Time (500s/Div)
Switching
Switching
VL1 (10V/Div) VOUT ac coupled (100mV/Div)
VL1 (10V/Div) VOUT ac coupled (100mV/Div)
IL1 (500mA/Div)
VIN = 3.3V, IOUT = 200mA, f = 640kHz
IL1 (500mA/Div)
VIN = 3.3V, IOUT = 200mA, f = 1.2MHz
Time (1s/Div)
Time (1s/Div)
Load Transient Response
Load Transient Response
VOUT ac coupled (500mV/Div)
VOUT ac coupled (500mV/Div)
ILOAD (200mA/Div)
VIN = 3.3V, f = 640kHz
ILOAD (100mA/Div)
VIN = 3.3V, f = 1.2MHz
Time (250s/Div)
Time (250s/Div)
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RT9277A/B
Application Information
Preliminary
The design procedure of Boost converter can start from the maximum input current, which is related about inductor, catch-diode input/output capacitor selections and the maximum power which internal switch can stand. It can be derived from maximum output power, minimum input voltage and the efficiency of Boost converter. Once the maximum input current is calculated, the inductor value can be determined and the other components as well. Inductor Selection For a better efficiency in high switching frequency converter, the inductor selection has to use a proper core material such as ferrite core to reduce the core loss and choose low ESR wire to reduce copper loss. The most important point is to prevent the core saturated when handling the maximum peak current. Using a shielded inductor can minimize radiated noise in sensitive applications. The maximum peak inductor current is the maximum input current plus the half of inductor ripple current. The calculated peak current has to be smaller than the current limitation in the electrical characteristics. A typical setting of the inductor ripple current is 20% to 40% of the maximum input current. If the selection is 40%, the maximum peak inductor current is :
IPEAK = IIN(MAX) + 1 IRIPPLE = 1.2 x IIN(MAX) 2 IOUT(MAX) x VOUT = 1.2 x x VIN(MIN)
Input Capacitor Selection For better input bypassing, low-ESR ceramic capacitors are recommended for performance. A 10F input capacitor is sufficient for most applications. For a lower output power requirement application, this value can be decreased. Output Capacitor Selection For lower output voltage ripple, low-ESR ceramic capacitors are recommended. The tantalum capacitors can be used as well, but the ESR is bigger than ceramic capacitor. The output voltage ripple consists of two components: one is the pulsating output ripple current flows through the ESR, and the other is the capacitive ripple caused by charging and discharging.
VRIPPLE = VRIPPLE_ESR + VRIPPLE_C - VIN I V IPEAK x RESR + PEAK OUT COUT VOUT x fOSC
Output Voltage The regulated output voltage is calculated by :
VOUT = VREF x 1 + R1 R2
Where VREF = 1.24V typical. For most applications, R2 is a suggested a value up to 100k Place the resistor-divider as close to the IC as possible to reduce the noise sensitivity. Loop Compensation
The minimum inductance value is derived from the following equation :
L=
x VIN(MIN) x [VOUT -VIN(MIN) ]
2 2
0.4 x IOUT(MAX) x VOUT x fOSC
Depending on the application, the recommended inductor value is between 2.2H to 10H. Diode Selection To achieve high efficiency, Schottky diode is good choice for low forward drop voltage and fast switching time. The output diode rating should be able to handle the maximum output voltage, average power dissipation and the pulsating diode peak current.
The RT9277A/B voltage feedback loop can be compensated with an external compensation network consisted of R3, C3 and C4 (As shown in Figure 1). Choose R3 to set the high-frequency integrator gain for fast transient response without over or under compensation. Once R3 is determined, C3 is selected to set the integrator zero to maintain loop stability. The purpose of C4 is to cancel the zero caused by output capacitor and the capacitor ESR. If the ceramic capacitor is selected to be the output capacitor, C4 can be taken off because of the small ESR. C2 is the output capacitor as shown in Figure 1. The following equations give approximate calculations of each component :
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Preliminary
R3 = 200 x VOUT 2 x C2 L1
RT9277A/B
-3 C3 = 0.4 x 10 x L1 VIN 0.005 x RESR x L1 C4 = VOUT 2
The best criterion to optimize the loop compensation is by inspecting the transient response and adjusting the compensation network. Soft-Start Capacitor The soft-start function begins from VSS = 0V to VSS = 1.24V with a 4A constant current charging to the soft-start capacitor, so the capacitor should be large enough to let the output voltage reach regulation inside the soft-start cycle. Typical value of soft-start capacitor range is from 10nF to 200nF. After the cycle finished, the load can start to draw maximum current as required. Layout Guideline For high frequency switching power supplies, the PCB layout is important step in system application design. In order to let IC achieve good regulation, high efficiency and stability, it is strongly recommended the power components should be placed as close as possible. These traces should be wide and short. The feedback pin and the networks of feedback and compensation should keep away from the power loops, and be shielded with a ground trace or plane to prevent noise coupling.
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RT9277A/B
Outline Dimension
Preliminary
D L
E
E1
e A A1 b A2
Symbol A A1 A2 b D e E E1 L
Dimensions In Millimeters Min 0.810 0.000 0.750 0.220 2.900 0.650 4.800 2.900 0.400 5.000 3.100 0.800 Max 1.100 0.150 0.950 0.380 3.100
Dimensions In Inches Min 0.032 0.000 0.030 0.009 0.114 0.026 0.189 0.114 0.016 0.197 0.122 0.031 Max 0.043 0.006 0.037 0.015 0.122
8-Lead MSOP Plastic Package
Richtek Technology Corporation
Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611
Richtek Technology Corporation
Taipei Office (Marketing) 8F, No. 137, Lane 235, Paochiao Road, Hsintien City Taipei County, Taiwan, R.O.C. Tel: (8862)89191466 Fax: (8862)89191465 Email: marketing@richtek.com
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DS9277A/B-02 March 2007

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