Product Specification
IXD3235/36/37
LAYOUT AND USE CONSIDERATIONS
1. Wire external components as close to the IC as possible and use thick, short connecting traces to reduce
the circuit impedance. Please, pay special attention to the VIN and GND wiring. Switching noise, which
occurs from the GND, may cause the instability of the IC, so, position VIN and VCL capacitors as close to IC
as possible.
2. Transitional voltage drops or voltage rising phenomenon could make the IC unstable if ratings are
exceeded.
3. The IXD3235/36/37 series are designed to work with ceramic output capacitors. However, if the difference
between input and output voltages is too high, a ceramic capacitor may fail to absorb the resulting high
switching energy and oscillation could occur. In this case, connect an electrolytic capacitor in parallel to
ceramic one to compensate for insufficient capacitance.
4. In PWM mode, IC generates very narrow pulses, and there is a possibility that some cycles will be skipped
completely, if the difference between VIN and VOUT is high.
5. If the difference between VIN and VOUT is small, IC generates very wide pulses, and there is a possibility
that some cycles will be skipped completely at the heavy load current.
6. When dropout voltage or load current is high, Current Limit may activate prematurely that will lead to IC
instability. To avoid this condition, choose inductor’s value to set peak current below Current Limit
threshold. Calculate the peak current according to the following formula:
IPK = (VIN - VOUT) x D / (2 x L x fOSC) + IOUT, where
L - Inductance
fOSC -- Oscillation Frequency
D – Duty cycle
7. Inductor’s rated current should exceed Current Limit threshold to avoid damage, which may occur until
P-channel transistor turns off after Current Limiter activates (see figure below).
Current flows into P-channel transistor reaches the current limit (ILIM).
Current is more than ILIM due the circuit’s delay time from the current limit detection to the P-channel transistor OFF.
The inductor’s current time rate becomes quite small.
IC generates very narrow pulses for several milliseconds.
The circuit latches, stopping operation.
8. If VIN voltage is less than 2.4 V, current limit threshold may be not reached due voltage drop caused by
switching transistor’s ON resistance
9. Latch time may become longer or latch may not work due electrical noise. To avoid this effect, the board
should be laid out so that input capacitors are placed as close to the IC as possible.
10. Use of the IC at voltages below recommended voltage range may lead to instability.
11. At high temperature, output voltage may increase up to input voltage level at no load, because of the
leakage current of the driver transistor.
12. High step-down ratio and very light load may be cause of intermittent oscillations.
13. In PWM/PFM automatic switching mode, IC may become unstable during transition to continuous mode.
Please verify with actual components.
PS034201-0515
PRELIMINARY
14
ZILOG [ ZILOG, INC. ]
