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SD46520
400KHZ 2A STEP-DOWN DC-DC CONVERTER
DESCRIPTION
The SD46520 is a step-down PWM control converter with a built-in internal power MOSFET. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. Current mode operation provides fast transient response and eases loop stabilization. It provides cycle-by-cycle current limiting and thermal shutdown. In shutdown mode, the current is only 23A.
SOP-8-225-1.27
FEATURES
* 2A output current * 0.2 internal power MOSFET * Stable with low ESR output ceramic capacitor * Up to 95% conversion efficiency * shutdown mode * 400kHz fixed frequency * Thermal shutdown * Cycle-by-cycle over current protection * 4.7518V input voltage range * 1.2216V output voltage range * Programmable under voltage control Device SD46520 Package Seal SOP-8-225-1.27 SD46520
ORDERING INFORMATION
APPLICATIONS
* Distributed power system * Battery charger * TFT LCD Monitors * Portable DVD * Set-Top Box
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SD46520
BLOCK DIAGRAM
2
IN Internal 3.3V Regulators + +
Current Sense Amplifier
CLK Oscillator 50/400kHz Shutdown comparator EN 1.8V
5V S Q
BS
1
R Current Comparator
Q
SW
3
7
1uA
2.30/2.53V
UVLO
Zero current control
GND
4
Frequency Foldback Comparator
0.7V FB
1.22V
Error Amplifier
COMP
5
6
ABSOLUTE MAXIMUM RATINGS
Characteristics Input Voltage Switch Voltage Boost Voltage Feedback Voltage Enable Voltage Comp Voltage Junction Temperature Lead Temperature Storage Temperature Symbol VIN VSW VBS VFB VEN VCOMP Tj TL Tstg Ratings 20 -1~VIN+1 VSW+6 -0.3~6 -0.3~6 -0.3~6 150 260 -65~150 Unit V V V V V V C C C
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SD46520
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, VIN=12V, Tamb=25C)
Characteristics Feedback Voltage Input Voltage Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Limit Current Limit Gain. Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance Oscillator Frequency Short Circuit Frequency Maximum Duty Cycle Minimum Duty Cycle Enable Threshold Voltage Enable Pull-up Current Under Voltage Lockout Threshold Voltage Under Voltage Lockout Threshold Hysteresis Voltage Supply Current (Quiescent) Supply Current (Operating) Thermal Shutdown IIN1 IIN2 TOT VEN0.4V VEN2.6V;VFB=1.4V ---23 3 180 36 3.5 -A mA C VUVLOH -210 -mV Av Gm fs fsshort Dmax Dmin VEN IENPUP VUVLO VFB=0V VFB=1.0V VFB=1.5V ICC>100A VEN=0V IC=10A -500 360 46 --0.7 1.5 2.37 400 770 400 58 90 -1.0 2.5 2.495 -1100 440 70 -5 1.3 3 2.62 V/V KHz KHz % % V A V GCS -1.95 -A/V Symbol VFB VIN RONH RONL ILEAK ILIM VEN=0V; VSW=0V Test Condition 4.75VIN18V VCOMP<2V Min. 1.220 4.75 ---2.4 0.2 10 0 2.85 Typ. 1.244 Max. 1.268 18 --10 3.3 Unit V V A A
PIN CONFIGURATION
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SD46520
PIN DESCRIPTIONS
Pin No. 1 2 3 4 5 6 7 8 Pin Name BS IN SW GND FB COMP EN NC I/O I/O I I/O G I I/O I I Bootstrap pin. Connect 10nF capacitor to SW pin. Input supply voltage. Switch pin. Connect with inductor. Ground. Feedback voltage input pin. Compensation pin. Enable input pin. No connection. Description
FUNCTION DESCRIPTIONS
SD46520 is a current mode DC-DC convertor with PWM control. The working process is as follows. At the beginning of a cycle, the switching MOSFET is off, the freewheeling MOSFET is on, SW is connected to the ground; the external bootstrap capacitor between BS and SW is charged by 5V voltage via internal Schottky diode. The bootstrap capacitor voltage is used as the power supply of the driver of switch MOSFET to make it work normally. When the bootstrap capacitor voltage is higher than the switching MOSFET's threshold voltage, the control loop circuit starts to work. The rising edge of the 400kHz clock signal sets the RS Flip-Flop. Its output turns on the switching MOSFET, the input supply is connecting to the output capacitor via the inductor, and the inductor current is increasing linearly and charges the output capacitor. The inductor current is sensed and amplified by the current sense amplifier. Ramp compensation is summed to current sense amplifier's output and compared to the error amplifier output by the current comparator. When the sum of current sense amplifier's output plus slope compensation signal exceeds the comp pin voltage, the RS Flip-Flop is reset. If the sum of current sense amplifier's output plus slope compensation signal does not exceed the comp voltage during one cycle, the falling edge of the CLK resets the Flip-Flop. After RS Flip-flop is reset, the switching MOSFET is off, the freewheeling MOSFET is on, and the external Schottky diode is on at the same time which carries most of the inductor current, the inductor current decreases linearly. When the next rising edge of the clock arrives, sets the RS Flip-flop again, the switching MOSFET is on, which is cycled and the output capacitor is charged to the output voltage setting. The voltage of COMP pin is the integral of the voltage difference between FB feedback voltage and 1.22V reference voltage. It is proportional to the peak inductor current. If COMP voltage increases, the inductor current and the output current also increase. When COMP voltage is rising to the high clamp voltage 2.3V, the output current is up to the limited value. Enable control Enable pin EN has the enable and under voltage lock two functions. When EN voltage is lower than 1V, the chip is off; when EN voltage is higher than 1V but lower than 2.495V, the chip is enable, while COMP is pulled down to the ground and the switching MOSFET is not working, so this is under voltage lock state; When EN voltage is
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SD46520
higher than 2.495V, the chip works normally. Short protection When the output voltage is short connected to the ground, FB feedback voltage is pulled down to the ground, and the oscillator frequency will decrease to 50kHz from normal 400kHz, which also decreases the inductor current average value, that is the output current is decreased.
APPLICATION NOTE
Output voltage setting As right figure shows, the output voltage is determined by the feedback resistors ratio of R1 and R2. Generally, R210K, R1 is decided by: R1=R2.(VO/1.22-1). Inductor selection When output current is large, the regulator is working under continuous current mode that is inductor current is continuous and will not decrease to zero. The inductor value will affect the ripple of inductor current, and the relationship between them is as follows:
L=
VO (VI - VO ) . VI fS I
Where, VI is input voltage, VO is output voltage, fs is on-off frequency, I is the peak-peak value of inductor current ripple. In general, I is no larger than 30% of the maximum output current, thus to decide the inductor value. At the same time, the peak inductor current should be less than 2.4A, the value is decided by:
ILMAX = IO +
1 1 V (V - VO ) . IL = IO + O I 2 2 VI fS L
Input capacitor selection The input current of step-down DC-DC is not successive, an additional input capacitor CI is needed to keep input voltage stable. The input capacitor should be low ESR, and ceramic capacitor is the best choice, tantalum capacitor or low ESR electrolytic capacitor is all right. The input capacitor value should be larger than 10F. Output capacitor selection The output capacitor CO is decided by the ripple requirement of output voltage. The output voltage ripple is decided by:
VO = IL (ESR +
1 ). 8fS C O
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SD46520
Where,
VO
is output voltage ripple,
IL
is inductor current ripple, ESR is the equivalent serial resistor of
output capacitor. The freewheeling diode selection Select the schottky diode as freewheeling diode to reduce the loss caused by forward voltage drop of the diode. At the same time, the maximum current of schottky should be higher than 2.4A. Compensation network The resistor-capacitor compensation network connected to COMP pin make sure the stability of the control loop circuit. As the right figure: The DC gain of the control loop is:
A VDC =
VFB A VEA GCS RL VO
Where, VFB is feedback voltage, 1.22V; VO is setting output voltage; AVEA is the voltage gain of error amplifier, 400V/V; GCS is the trans-conductance of current sense(about the value of the output current divided by COMP pin voltage), 1.95A/V; RL is load resistor. The control loop has two poles of inportance. The first pole P1 is decided by the following formula:
fP1 =
1 2 R OEA CC1
,
where,
R OEA =
A VEA GEA
, GEA is the trans-conductance of error amplifier, 770uA/V.
The second pole P2 is decided by:
fP2 =
1 2 RL CO
1 2 R C CC1
The control loop has a main zero Z1, which is decided by: fZ1 =
When the output capacitor ESR is large, another zero Z2 is introduced which is decided by:
fZ2 =
1 . 2 ESR CO
At this time, it needs to add another compensation capacitor Cc2 to introduce an additional pole P3 compensate this zero , and this pole is decided by: f = P3
1 2 R C CC2
The system crossover frequency fC is very important. If it's too high , it will cause the system unstable. If it's too low, it will slower the system transient response. In general, fC is about 1/8~1/10 of the switch frequency. Use the following procedure to choose the value of the compensation components. First, according to the selected crossover crossover fC, choose the value of compensation resistor Rc:
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SD46520
RC = 2 CO VO fC GEA GCS VFB
.
Second, select Z1 as about 1/5~1/4 of crossover frequency to confirm the compensation capacitor Cc1 value: CC1 =
1 . 2 R C fZ1
When compensation capacitor Cc2 is needed, the value of it is decided by:
CC2 =
ESR CO RC
.
TYPICAL APPLICATION CIRCUIT
SD46520
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SD46520
PACKAGE OUTLINE
SOP-8-225-1.27 UNIT: mm
MOS DEVICES OPERATE NOTES:
Electrostatic charges may exist in many things. Please take following preventive measures to prevent effectively the MOS electric circuit as a result of the damage which is caused by discharge: The operator must put on wrist strap which should be earthed to against electrostatic. Equipment cases should be earthed. All tools used during assembly, including soldering tools and solder baths, must be earthed. MOS devices should be packed in antistatic/conductive containers for transportation.
Disclaimer: * Silan reserves the right to make changes to the information herein for the improvement of the design and performance without further notice! * All semiconductor products malfunction or fail with some probability under special conditions. When using Silan products in system design or complete machine manufacturing, it is the responsibility of the buyer to comply with the safety standards strictly and take essential measures to avoid situations in which a malfunction or failure of such Silan products could cause loss of body injury or damage to property. Silan will supply the best possible product for customers!
*
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