"In circuit board design, it is often necessary to provide various levels of voltage for multiple chips, and with the improvement of chip integration, the power supply is required to have higher dynamic performance. Therefore, non isolated point of load DC-DC applications are more and more. These load point power modules provide separate power supply voltages of different levels for devices on the circuit board. There are many kinds of DC-DC converters on the market. Today, Aiban network brings you an evaluation of isl85415, a Synchronous Step-Down regulator of INTERSIL company. INTERSIL has always focused on the design of analog mixed signal and power management IC. This newly released isl85415 is a wide input Synchronous Step-Down regulator, and integrates FET switch to reduce peripheral circuits and make the design simpler.
When INTERSIL released the Synchronous Step-Down regulator isl85415, it also launched two evaluation kits isl85415eval1z and isl85415demo1z at different prices. Among them, isl85415eval1z has more comprehensive evaluation functions and the price is $72; The isl85415demo1z is a simplified demo version that costs only $20. The evaluation board can more intuitively feel the power conversion function of isl85415 and the simplicity of design. Aiban network got the Evaluation Kit at the first time. However, let's first understand the characteristics of isl85415 chip.
Isl85415 is actually a switching DC-DC voltage regulator. We know that switching regulators can generally be divided into three forms: Buck, boost and buck boost.
Buck basic circuit
Isl85415 is a step-down switching regulator, and integrates high side and low side FETs. In this way, the external FET or diode of the peripheral circuit can be omitted, which not only reduces the peripheral circuit design, but also reduces the size of the whole power circuit. The following figure is a typical application block diagram of isl85415. The peripheral circuit only needs inductance and several resistors and capacitors.
Isl85415 typical applications
As an efficient wide input Synchronous Step-Down regulator, isl85415 has the following other characteristics:
Wide input voltage range VIN 3v-36v
Output voltage range Vout 0.6V to 95% of VIN (up to 34V output if 36V input)
Maximum continuous output current 500mA
Support synchronous operation to achieve greater conversion efficiency
Built in 500Hz fixed working frequency or optional 300hz-2mhz adjustable working frequency
Internal integrated compensation circuit, or external compensation circuit can be selected
PFM mode can be selected under light load to improve efficiency
Internal or external soft start signal (soft start)
Power good signal indication and chip enable signal
4mm x 3mm 12 pin DFN package
It can be seen that isl85415 provides a very flexible solution. The wide input voltage range of 3v-36v and flexible output can meet the needs of many industrial applications and battery power supply. Internal integrated functions such as integrated compensation circuit, support for internal soft start and built-in switching frequency can simplify the design of power circuit, and various interfaces can also meet more complex design. The typical circuit using isl85415 only needs to adjust the device parameters of the external circuit to design different output voltage schemes.
A major feature of isl85415 is that PFM (pulse frequency modulation) conversion mode can be adopted in light load. We know that switching power supply can generally achieve high conversion efficiency under high load. Many devices take the maximum efficiency as the parameter, and some can reach more than 95%. However, often the circuit does not work under high load, which may be the opposite, so the efficiency under light load may be more important. Isl85415 can automatically enter PFM mode under light load, effectively improving efficiency; At high load, it can automatically enter PWM (pulse width modulation) mode to reduce output ripple. This pfm-pwm switching control can effectively improve the overall efficiency of power supply circuit.
Under light load, PFM mode maintains high efficiency, while PWM mode efficiency decreases sharply
After understanding some of the main features of isl85415, let's see what experience its evaluation board can bring.
The isl85415eval1z evaluation board has more test interfaces and peripheral control circuits than the simplified isl85415demo1z demonstration board. The power module circuits built around isl85415 on the board are exactly the same. Their prices are still quite different, and those who are interested can choose according to their own needs. Here we mainly introduce the isl85415eval1z evaluation board.
Isl85415eval1z evaluation board is used for the evaluation of power chip, and the circuit is not complex. The workmanship of the evaluation board is very good and the materials are sufficient, but the price is also very expensive. The quotation on the official website is $72. The evaluation board adopts the red 4-layer circuit board design, and the whole circuit board size is 69mm x 78mm square circuit board. There is also a row of "golden finger" test interfaces on one side of the evaluation board. The core part of the evaluation board is the reference circuit of the power module composed of isl85415, which only occupies 1.516 For the area of cm2 and BOM list, please refer to the user manual of evaluation version.
The reference circuit area of isl85415 and its periphery is 0.5 "x0.47", which is about 1.516cm2
Isl85415eval1z evaluation board provides various test points and interfaces to facilitate experiment and demonstration evaluation, such as tower terminal of input and output power supply, special oscilloscope probe test points and switches, etc
P4, P5: input power terminal
P7, P9: output power terminal
J1 and J2: probe test points (switching frequency and output voltage)
SW1: enable switch
SW2: PFM / PWM selector switch
P1：Power-Good
P2: sync synchronization signal
P6: enable signal input
P8: internal 5V bias voltage
P10：Soft-Start
The peripheral composition circuit of isl85415 is relatively simple and does not need external FET or diode. It mainly includes voltage dividing resistance, capacitance, inductance and other elements. The following is the power conversion circuit composed on isl85415 evaluation board. The circuit is designed according to the fixed output voltage of 3.3V.
It can be seen that there is a 150uf electrolytic filter capacitor at the power input, which can better improve the input power supply. Note that resistance bits that can be short circuited with 0 Ω resistance are reserved in SS, FS and comp respectively.
R15 is short circuited, then the circuit adopts the internal soft start signal of isl85415, otherwise the rise time of soft start signal is controlled by CSS capacitor;
R10 is short circuited. The fixed switching frequency of the voltage regulator is 500KHz, otherwise the frequency set by R12 (300khz-2mhz);
If R11 is short circuited, the circuit adopts the internal compensation of the voltage regulator, otherwise the external compensation circuit is used.
These resistors equivalent to switches increase the flexibility of the evaluation board and can more completely test all functions of isl85415. For detailed introduction, please refer to the device data manual.
Of course, the evaluation board can also output other amplitude voltages. The output voltage can be changed by adjusting the voltage dividing resistance connected to the FB signal. For specific calculation parameters, please refer to the user manual and device data manual.
The voltage dividing resistance of the circuit is troublesome to disassemble and replace. If it can be realized by potentiometer, it will be more convenient to adjust, but the size of the power module circuit should also be considered. Less gossip, take the circuit output of the evaluation board as 3.3V. Now let's power on and try the performance of isl85415. Before we start, we need some equipment:
One DC power supply
An electronic load
A digital multimeter
An oscilloscope
One signal generator (if you want to test the external synchronous clock)
First, the following simple connection circuit is connected here, only the input power supply is connected, and the switching frequency (phase) and output voltage are tested (tested with oscilloscope), and the output voltage is connected to the electronic load. The isl85415eval1z evaluation board is specially equipped with two special oscilloscope probe test points (TEK 131-4353-00) to test the switching frequency and output voltage. This test point is very convenient to use. Insert the probe directly, especially the ordinary probe is only needed to measure the voltage ripple.
After power on, the evaluation board starts to work. The output voltage of the evaluation board is fixed at 3.3V, so the input voltage between 3.3v-36v can work normally. Adjust the input voltage between 3.3v-36v, you can see that the LED indicator on the board lights up and shows that the power good signal output is normal.
When the voltage is less than 3.3V, the PG signal becomes low and the LED goes out. Although it can continue to work when the input voltage is slightly lower than 3.3V and the output voltage is lower than 3.3V, the work is unstable at this time, the PG signal has become low, continue to reduce the input voltage and the chip stops working completely.
The SW1 switch on the board controls the enable signal and can control the opening and closing of isl85415.
SW2 switch controls the working mode of isl85415 voltage regulator. In "PFM" position, the voltage regulator can enter PFM mode under light load and PWM mode under high load; In "PWM" gear, the forced voltage regulator works completely in PWM mode, and at this time, we can synchronize the switching frequency through P2 external synchronization clock.
PFM operating mode
Now we turn the SW2 switch to the "PFM" position, turn on the electronic load, and first set the load current to 0A (equivalent to no-load). Turn on the power supply (5V), isl85415 enters PFM working mode, and increase the electronic load current to 010ma. It can be seen that the switching frequency is discontinuous and the output ripple is 15.2mv.
Light load (10mA), PFM mode, input voltage 4.9v, output voltage 3.3V, ripple 15.2mv (click to enlarge the picture)
Continue to increase the load current, the switching frequency waveform changes constantly, and the cut-off time becomes shorter and shorter. After the load current increases to 150mA, it enters the PWM mode. It can be seen that the switching frequency is a 769khz square wave (set by R12 resistance). At this time, the ripple becomes smaller, only a few millivolts (2.4mv).
Load 270ma, PWM mode, input voltage 4.9v, output voltage 3.3V, ripple 2.4mv (click to enlarge the picture)
The maximum continuous current parameter that isl85415 chip manual can provide is 500mA. When we keep increasing the load current, the voltage regulator circuit can still work until the load current exceeds 840ma, the output of isl85415 starts to turn off, the power good signal becomes low, and the LED on the evaluation board goes out. This is the overcurrent protection function of the chip. After reducing the load, the chip can work normally again.
Load 840ma, overcurrent protection (click to enlarge the picture)
Forced PWM operating mode
When the SW2 switch is set to "PWM", the voltage regulator isl85415 is forced to enter the PWM working mode. It can be seen that the switching frequency is also a 769khz square wave under light load (current 10mA), and the ripple at this time is only 1.6mv. Because the switch is constantly switched in PWM mode, unlike the complete cut-off in PFM mode, the efficiency is reduced.
Light load (10mA), PWM mode, input voltage 4.9v, output voltage 3.3V, ripple 1.6mv (click to enlarge the picture)
The output voltage of isl85415eval1z is fixed at 3.3V. When we increase the input voltage, the output voltage remains unchanged (limited by conditions, measured by oscilloscope), and the duty cycle and voltage of phase square wave will change.
Maximum load 500mA, Input Voltage 5V, output 3.3V
Maximum load 500mA, input voltage 28V, output 3.3V
Power on start sequence
The voltage regulator isl85415 supports soft start, which can also meet more and more applications requiring power on timing. R15 on the isl85415eval1z evaluation board is equipped with a 0 Ω resistor, so the chip on the evaluation board adopts an internal soft start signal, about 2ms.
Waveform at power on time when the input voltage is 5V and the maximum load is 500mA
Input Voltage 5V, no-load, power on time waveform
Isl85415 evaluation board also provides some other test points to more comprehensively test the performance of this Synchronous Step-Down regulator. The power standard reference circuit of the evaluation board gives us more choices. We can even directly realize other functions or output other levels of voltage on the evaluation board. In this process, we only need to replace some resistors or capacitors.
Summary
The wide input Synchronous Step-Down regulator isl85415 newly introduced by INTERSIL company is introduced. INTERSIL launched two boards simultaneously, including the $72 isl85415eval1z evaluation board and the $20 isl85415demo1z demonstration board. Isl85415 has a very wide input range (3v-36v) and integrates high side and low side FETs to reduce peripheral circuits and improve stability. Moreover, it integrates functions such as internal soft start, compensation circuit, enable control, external synchronous input and so on. The design of the whole power module becomes very convenient and flexible. It is very suitable for industrial applications with high voltage and some battery powered devices.
The evaluation board of isl85415 is easy to use and can quickly experience the switching performance of voltage regulator. The power reference circuit of the evaluation board can be used as a standard reference, and its area is only 1.516cm2. The test interface provided by the evaluation board, especially the special probe interface, greatly facilitates the signal test. If you want to learn more about this simple and flexible switching regulator, please visit INTERSIL's official website. If you want to experience it immediately, you can apply for free demo board samples. The number is limited and the opportunity is rare!
reference material
INTERSIL wide input integrated FET Synchronous Step-Down regulator isl85415
Intersil isl85415eval1z evaluation board
Intersil isl85415demo1z demo board
Creative DIY, fire fighting isl85415demo1z
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