ISL9239
Dual-input synchronous buck controller

What it is

The ISL9239 is a Renesas dual-input synchronous buck converter controller designed to manage power delivery from multiple sources—typically USB Power Delivery and legacy USB-C barrel connector inputs. It's found on modern Apple MacBook, iPad, and gaming console boards where multi-source charging needs to be seamlessly arbitrated.

The chip controls two independent input channels, monitors voltage and current on each, and automatically selects the source with highest available power. It's responsible for maintaining PPBUS rail stability during fast charging, controlling 5V, 9V, 15V, and 20V USB PD negotiation, and driving the MOSFETs that switch power onto the main rail.

The ISL9239 communicates via I2C with the power management IC and includes integrated droop compensation, temperature monitoring, and current limiting. It's a QFN-40 (5×6mm) package with analog and digital I/O pins for feedback, GATE drive outputs, and telemetry.

Critical pins

Input channels: VIN1 and VIN2 sense the two input rails. Typically VIN1 monitors the USB-C port input, VIN2 monitors legacy barrel connector or secondary source.

Gate drivers: GATE1 and GATE2 output PWM signals (swinging 0–5V logic) to drive the upper and lower MOSFETs of the synchronous buck stage. Rise time is critical—slow gate edges cause massive switching losses and thermal issues.

Feedback: FB pin measures the output voltage via resistive divider. Expected DC bias around 0.5–1.5V under normal operation, representing the actual PPBUS voltage. Resistor network failures here cause voltage regulation collapse.

Sense resistor: IOUT_SNS measures current through a small series resistor (typically 10mΩ) in the output path. This analog voltage (millivolts range) is used for current limiting and telemetry.

I2C: SCL and SDA carry bus voltage around 1.8V or 3.3V (with pull-ups). Communicates charging parameters, input power status, and temperature to the system controller.

Typical measurements

In practice

Charging failure: Board won't charge or charges extremely slowly. Probe FB with your meter. If it's stuck at 0V, the feedback divider resistors are likely open or the ISL9239 is not responding to feedback. If it's stable but PPBUS is low (under 15V), the ISL9239 isn't properly regulating—check GATE1/GATE2 for PWM activity.

No USB PD negotiation: Charging cable plugged in but system doesn't wake or accept power. This often means the ISL9239 I2C interface is dead. Probe SCL and SDA for stuck-low conditions. If both lines are held low, there's a short to GND on the bus, or the chip itself is shorted.

Thermal runaway: Board gets extremely hot during charging. The ISL9239 is sending continuous gate drive to the MOSFETs, creating massive switching loss. Check if GATE1/GATE2 are oscillating at reasonable frequency (hundreds of kHz). If they're at DC or very low frequency, the feedback loop is broken.

Intermittent power loss: System loses power randomly under load. The IOUT_SNS line may be floating or the sense resistor is cracked. Current limiting would be disabled, causing the chip to shut down when it detects an internal over-temperature. Measure resistance across the sense resistor (should be ~10mΩ). If open, replace the 10mΩ component.

Common failures

Shorted chip (VCC to GND): ISL9239 failure typically manifests as dead I2C bus. Test resistance from VCC to GND—if below 100Ω, the chip is internally shorted. This usually happens due to ESD damage on the VIN1 or VIN2 pins during rough charging plug insertion.

Blown gate drivers: One or both GATE pins stuck low. The internal MOSFET driver stage has failed. The buck converter won't operate. You'll need to replace the entire chip (it's not field-repairable).

Feedback divider resistor network open: The two resistors forming the FB divider (typically 20kΩ and 10kΩ from output to FB to GND) crack under thermal cycling. FB floats to intermediate voltage, feedback is lost, and regulation collapses. Measure with resistor mode on both divider resistors. Replace if either reads open.

I2C pull-up resistor failure: SCL or SDA won't go high. The 4.7kΩ (typical) pull-up to 3.3V is open. I2C communication halts. Test by measuring resistance from each line to its rail—should be ~4.7kΩ when no device is pulling it low.

Repair strategy

Step 1: Verify PPBUS under load. With a USB PD charger plugged in, measure the main power rail. If it stays above 16V for 5 seconds, the ISL9239 is likely functional. If it collapses immediately, move to step 2.

Step 2: Check feedback path. Probe the FB pin. If it's held at 0V, the chip is not responding. Check the I2C bus first (step 3). If FB varies between 0.8–1.2V but PPBUS is still low, the feedback resistor divider may be open.

Step 3: Test I2C interface. Use a bus analyzer or scope on SCL and SDA. Apply a small pull-down (10kΩ resistor to GND) to each line and release. If the line doesn't spring back to 3.3V, the pull-up is failed or the ISL9239 is clamping it. If both lines are stuck low, ISL9239 is shorted internally.

Step 4: Measure gate drive activity. Scope GATE1 under load. You should see a square wave at 100–400 kHz switching frequency. If it's DC (always high or always low), the buck loop is not oscillating. This usually means the chip is non-functional.

Step 5: Replace the IC. ISL9239 failures are rarely marginal—the chip either works or it doesn't. If all the above tests point to the chip being dead, reballing or replacing the QFN is required. This is not a field repair for most technicians; it requires BGA rework equipment.

See also

Related terms in this glossary:

• ISL6259 — Simpler single-input synchronous buck controller, used on older designs
• USB Power Delivery — Protocol the ISL9239 implements for voltage negotiation
• PPBUS_G3H — Main output rail typically controlled by this chip
• I2C — Communication bus used by ISL9239 for telemetry and control