Features

Input Voltage: 3.6V to 18V ; Output Voltage: 0.8V to VCC; Output Current: Up to 3A; Duty Cycle: 0% to 99% PWM Control; Oscillation Frequency: Typical 300kHz; Soft-Start, Current Limit, and Enable Features; thermal shutdown function; built-in sw p channel mos; SOP-8L and SOP-8L-DEP: with "green" molding compound (no BR, SB) 8226 ; lead free finish/RoHS compliant.

General Instructions

The AP1538 consists of a step-down switching regulator with pwm control. These devices include reference voltage source, oscillator circuit, error amplifier, internal pmos.

AP1538 has low ripple power, high efficiency and good transient characteristics. The pwm control circuit can linearly change the duty cycle from 0 to 99%. The converter also contains an error amplifier circuit and a soft-start circuit to prevent overshoot during start-up. Built-in enable function, overcurrent protection function and short circuit protection function, when OCP or SCP occurs, the operating frequency is reduced from 300kHz to 50kHz. Additionally, an internal compensation block is built in to minimize external component counts.

By adding internal p-channel power mos, coils, capacitors, and externally connected diodes, these ICs can act as step-down switching regulators. When combined with the sop-8l and sop-8l-dep miniature packages, they are ideal power supply units for portable devices with outstanding features such as low current consumption. Since the converter can accommodate input voltages up to 18V, it is also suitable for operation via an AC adapter.

application

PC motherboards; LCD monitors; graphics cards; DVD video players; telecommunications equipment; ADSL modems; printers and other peripherals; microprocessor core power supplies.

Typical application circuit:

Note: 6. Recommended diode power Schottky P/N: B340 series or PDS340 ; 7. Recommended voltage <1V; 680 μF; 8. Typical feedback compensation (CC): 5600pF.

Typical performance characteristics:

Function description

PWM control

The ap1538 is a dc/dc converter that uses a pulse width modulation (pwm) scheme. Its pulse width varies from 0% to 99%, depending on the output current load. The output ripple voltage caused by pwm high frequency switching can be easily reduced by an output filter. Therefore, the converter provides low-ripple output power over a wide range of input voltage and output current loads.

undervoltage lockout

The AP1538's undervoltage lockout circuit ensures that the high-side MOSFET driver remains off when the supply voltage drops below 3.3V. Normal operation resumes when VCC rises above 3.5V.

Current limiting protection

The current limit threshold is set by an external resistor rocset connected from the vcc supply to the ocset pin. The voltage of the ocset pin is set by the internal leakage current iocset of this resistor (typical value is 90ua). When the pwm voltage is less than the voltage of ocset, the overcurrent condition is triggered.

The current limit threshold is given by:

ipeak is the output peak current; rds(on) is the on-resistance of the mosfet; fs is the pwm frequency (typically 300khz). In addition, the inductance value will affect the ripple current Δi.

The above formula applies to the input voltage range of 5V to 18V. A higher ROCSET is recommended for input voltages below 5V or for input voltages with ambient temperatures exceeding 100°C.

Sensor selection

For most designs, use 22µh to 33µh inductors for operation. The inductor value can be derived from the following equation:

where Δil is the inductor ripple current. Large value inductors lead to low ripple current and small value inductors result in high ripple current. The inductor ripple current is chosen to be approximately 15% of the maximum load current of 3A, IL = 0.45 A. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation (3A + 0.225A).

Input Capacitor Selection

This capacitor should use short leads close to the IC and should be rated for approximately 1.5 times the maximum input voltage. The RMS current rating of the buck regulator's input capacitors is approximately 1x2 DC load current. A low ESR input capacitor must be used, subject to the maximum RMS current. For most applications, a low ESR capacitor of 470µF is sufficient.

Output Capacitor Selection

An output capacitor is required to filter the output voltage and provide stability of the regulator loop. Important capacitance parameters are 100kHz Equivalent Series Resistance (ESR), rms ripple current rating, voltage rating and capacitance value. For the output capacitor, the esr value is the most important parameter. The output ripple can be calculated by the following formula.

The esr of the bulk capacitor will determine the output ripple voltage and the initial voltage drop after high slew rate transients.

The esr value of aluminum electrolytic capacitors is related to the capacitance and its rated voltage. In most cases, high voltage electrolytic capacitors have lower esr values. In most cases, capacitors with higher voltage ratings may be required to provide the low esr values required for low output ripple voltage.

Printed Circuit Board Layout Guidelines

If you need low TC and TJ or large PD (power dissipation), the SOP-8L package on dual SW pins (5 and 6) and VSS pins (7 and 8) internally connected to die pads, the evaluation board should allow the Output (SW) pin maximum copper area.

1. Connect the FB circuit (R1, R2, C1) as closely as possible, away from the inductive flux of pure VFB.

2. Connect C3 with VCC and VSS pins as closely as possible to get a good power filter effect.

3. Connect R4 to the VCC and OCSET pins as close as possible.

4. Connect the ground sides of C2, D1, and C4 as closely as possible, and use a ground plane for best performance.

Function Description: