CJ7912 - -12V Negative Linear Regulator

Fixed negative voltage regulator providing stable -12V output in compact TO-252-2L (DPAK) surface-mount package for negative power rail applications.

• View on JLCPCB: C94173

• Download Datasheet (PDF)

Overview

The CJ7912 is a three-terminal negative voltage regulator in TO-252-2L (DPAK) package, designed to provide a fixed -12V output from a more negative input voltage. This component serves as the final stage in the -12V power rail, converting the -13.5V DC-DC output to a clean, low-noise -12V suitable for modular synthesizer op-amp circuits and analog signal processing.

Key Specifications

Pin Configuration

         TO-252-2L (DPAK) Package
              Top View

         ┌──────────────┐
         │              │
         │    CJ7912    │
         │              │
         │              │
         │              │
         └──┬───────┬───┘
            │       │
          PIN1    PIN2
           GND    INPUT
        (Common) (-13.5V)

              TAB
           (OUTPUT)
            (-12V)

         Side View

         ┌──────────────┐
         │   Component  │ ← Surface mount IC
         └──┬───────┬───┘
            │       │
          PIN1    PIN2

    ════════════════════  ← Metal tab (OUTPUT)
                            soldered to PCB pad

Pin Descriptions

Critical Note: This is a NEGATIVE voltage regulator. Pin numbering and voltage polarities are different from positive regulators:

• Pin 1 is GND (not INPUT like 78xx series)

• Pin 2 is INPUT (not GND like 78xx series)

• More negative voltage is "higher" voltage for this regulator

• Input must be more negative than output (-13.5V is "higher" than -12V)

Application Circuit

-13.5V (from DC-DC) ──┬─── C13: 470nF ───┬─── U8: CJ7912 ────┬─── C16: 100nF ───┬─→ -12V OUT
                      │                  │                   │                  │
                      │                  │    ┌──────────┐   │                  │
                      │                  └────│2  INPUT  │   │                  │
                      │                       │          │   │                  │
                      │                    ┌──│1   GND   │   │                  │
                      │                    │  │          │   │                  │
                      │                    │  │   TAB ───┴───┴──────────────────┤
                      │                    │  └──────────┘                      │
                      │                    │     (OUTPUT)                       │
                      └─── C21: 470µF ─────┼─────────┬─── C22: 470µF ───────────┤
                           (Input)         │         │    (Output)              │
                                           │         │                          │
                                          GND       GND                         │
                                                                                │
                                        ┌───────────────────────────────────────┘
                                        │
                                  ┌─────┴─────┐
                                  │   LED4    │ Red Status LED
                                  │   (Red)   │ via R9: 1kΩ
                                  └─────┬─────┘
                                        │
                                       GND

Component Values

Input Capacitors

Important: For C21, negative terminal connects to -13.5V input, positive terminal connects to GND.

Output Capacitors

Important: For C22, negative terminal connects to -12V output, positive terminal connects to GND.

Status LED Circuit

LED Connection: Anode to GND, Cathode to -12V through R9 (reverse of positive rail LEDs).

Design Considerations

Negative Voltage Concepts

Understanding negative voltage regulators:

Voltage Levels (relative to GND = 0V):

                GND (0V) ─────────────────
                  │
                  │ +13.5V "above" ground
                  │
              +13.5V ─────────────────

                GND (0V) ─────────────────
                  │
                  │ 12V "below" ground
                  │
              -12V ─────────────────
                  │
                  │ 13.5V "below" ground
                  │
              -13.5V ─────────────────

For negative regulator:
- INPUT = -13.5V (more negative)
- OUTPUT = -12V (less negative)
- Dropout = |VIN| - |VOUT| = 13.5V - 12V = 1.5V

Input Voltage Requirements

The CJ7912 requires input voltage to be at least 2V more negative than the output for proper regulation:

• Input voltage: -13.5V from LM2596S DC-DC converter

• Output voltage: -12V

• Dropout margin: 13.5V - 12V = 1.5V

• Status: ⚠️ Marginal - operates near minimum dropout voltage

The 1.5V margin is slightly below the typical 2V dropout specification but acceptable because:

1. The DC-DC stage is regulated at -13.5V

2. The CJ7912 can regulate with 1.5V dropout at lower currents

3. Actual load current (0.8A) is well below maximum rating (1.5A)

4. Negative regulators often have slightly lower dropout than positive equivalents

Recommendation: For production, consider increasing DC-DC output to -14.0V for better dropout margin.

Thermal Management

Power dissipation calculation:

P = (|VIN| - |VOUT|) × IOUT
P = (13.5V - 12V) × 0.8A
P = 1.5V × 0.8A
P = 1.2W

Temperature rise without additional heatsinking:

ΔT = P × θJA
ΔT = 1.2W × 40°C/W
ΔT = 48°C

At 25°C ambient, junction temperature = 73°C (well within 125°C maximum).

Thermal performance of TO-252-2L:

• Moderate θJA (40°C/W)

• Metal tab provides good heat spreading

• Direct thermal contact with PCB copper pour

• No additional heatsink required for this application

PCB copper area recommendations:

• Minimum: 3 cm² copper pour connected to tab

• Recommended: 6 cm² copper pour for better margin

• Thermal vias: 6-8 vias (0.3mm) under tab to bottom layer

Capacitor Placement and Polarity

Critical for negative regulators: Electrolytic capacitor polarity is REVERSED from what you might expect:

Correct polarity for negative rail:

C21 (Input, 470µF):
         ┌─────────┐
  GND ───┤+       -├─── -13.5V
         └─────────┘

C22 (Output, 470µF):
         ┌─────────┐
  GND ───┤+       -├─── -12V
         └─────────┘

REMEMBER:
- Positive terminal goes to GROUND (0V)
- Negative terminal goes to negative voltage
- This is OPPOSITE of positive voltage regulators

Placement guidelines:

1. C13 (470nF ceramic): Place within 5mm of pin 2

• Purpose: Suppress high-frequency noise from DC-DC stage

• Non-polarized, so no polarity concerns

2. C21 (470µF electrolytic): Place within 10mm of pin 2

• Purpose: Stabilize input voltage during load transients

• POLARITY: + to GND, - to -13.5V

3. C16 (100nF ceramic): Place within 5mm of output tab

• Purpose: High-frequency output decoupling

• Critical for preventing oscillation

4. C22 (470µF electrolytic): Place within 10mm of output tab

• Purpose: Improve load transient response

• POLARITY: + to GND, - to -12V

Ground Plane Connection

The TO-252-2L package layout:

• Pin 1 (GND): Wide connection directly to ground plane

• Pin 2 (INPUT): Connect to -13.5V rail

• Tab (OUTPUT): Large copper pour for -12V distribution and thermal management

• Thermal vias: Essential for heat dissipation to inner/bottom layers

Performance Characteristics

Regulation Performance

Noise Performance

Protection Features

Built-in Protections

1. Thermal Shutdown: Automatically shuts down if junction temperature exceeds 150°C

2. Short Circuit Protection: Current limiting prevents damage during output short

3. Safe Operating Area (SOA): Internal circuitry ensures operation within safe limits

External Protection (This Design)

-12V (from U8) ──┬─── PTC3: 1.1A ──┬─── F3: 1.5A ──┬─── TVS3: SMAJ15A ───┬─→ -12V OUT
                 │  (Auto-Reset)   │   (Backup)    │  (15V Clamp Rev)    │
                 │                 │               │      ↑              │
                 │                 │               └─────GND─────────────┤
                 │                 │                                     │
                 └─── LED4 (Red) via R9 (1kΩ) ──────────────→ Power Status
                      (Cathode to -12V, Anode to GND through resistor)

TVS Diode Connection for Negative Rail:

         TVS3 (SMAJ15A)
            ┌─────┐
-12V ───────┤  │  ├────── GND
            └──▲──┘
               │
          (Cathode to -12V
           Anode to GND)

Protection stages:

1. Overload (0.9A-1.5A): PTC3 trips → Auto-reset after cooling

2. Short circuit (>1.5A): F3 blows → Manual replacement required

3. Overvoltage: TVS3 clamps transients (reverse-biased for negative voltage)

Note: For the negative rail, the TVS diode is connected with cathode to -12V and anode to GND, which is reverse of positive rail configuration.

Bill of Materials

Note: Higher total cost than positive regulators due to larger electrolytic capacitors (25V rating vs 10V).

Alternative Parts

Direct Replacements (TO-252-2L Package)

Package Alternatives

Stock Note: If C94173 (CJ7912) is out of stock, consider C192101 (L7912CV TO-220) which has similar availability.

PCB Layout Guidelines

Footprint Requirements

TO-252-2L (DPAK) package footprint specifications:

         Top View (PCB Pad Layout)

    ┌──────────────────────────────┐
    │                              │
    │    Large Copper Pour         │  ← OUTPUT TAB (-12V)
    │    (6cm² recommended)        │     thermal + electrical
    │                              │
    │         Thermal Vias         │
    │         (6-8 vias)           │
    │                              │
    └──────────────────────────────┘

         PIN1 ■        PIN2 ■
         (GND)       (INPUT)
                    (-13.5V)

    Pin spacing: 2.28mm
    Pin pad: 1.5mm x 2.0mm
    Tab pad: 10mm x 10mm (minimum)
            12mm x 12mm (recommended)

Recommended Pad Dimensions

Layout Recommendations

1. Component placement:

• Orient IC with tab facing interior of PCB

• Maximize copper area under and around tab for -12V rail

• Keep input and output capacitors on same side as regulator

• Separate -12V copper pour from +12V pour (clearance >2mm)

2. Copper pours:

• Create dedicated copper pour for -12V rail (6cm² minimum)

• Top layer: Main output pad and distribution to -12V loads

• Bottom layer: Additional copper connected via thermal vias

• Keep -12V copper separate from positive voltage pours

3. Thermal vias:

• Place 6-8 thermal vias (0.3mm diameter) under tab

• Arrange in grid pattern for even heat distribution

• Connect to bottom layer copper pour for -12V

• Direct connection (no thermal relief) for best heat transfer

4. Trace widths:

• Input trace (-13.5V): 0.8mm minimum (0.8A current)

• Output trace (-12V): 1mm minimum (0.8A current)

• Ground: Maximum copper pour area

• High-current paths: 2mm or copper pour preferred

5. Electrolytic capacitor polarity markings:

• Add clear silkscreen markings: "+" toward GND

• Add polarity indicators to prevent assembly errors

• Consider adding text: "NEG RAIL - CHECK POLARITY"

Critical Layout Warnings

⚠️  ELECTROLYTIC CAPACITOR POLARITY WARNING ⚠️

For negative voltage regulator:
  Positive terminal (+) → GND (0V)
  Negative terminal (-) → Negative voltage

This is OPPOSITE of positive regulators!

Add clear silkscreen markings to prevent
assembly errors that will destroy capacitors.

Thermal Via Pattern

    Recommended thermal via pattern
    under TO-252-2L tab:

    ┌─────────────────────────┐
    │                         │
    │  ●    ●    ●    ●    ●  │
    │                         │  ← 0.3mm vias
    │  ●    ●    ●    ●    ●  │     2.5mm spacing
    │                         │
    └─────────────────────────┘

    Total vias: 10 (aggressive cooling)
    Minimum: 6 vias
    Recommended: 8 vias

Assembly Considerations

Critical Assembly Warnings

ELECTROLYTIC CAPACITOR POLARITY:

The most common assembly error with negative voltage regulators is installing electrolytic capacitors backwards. This will cause:

• Immediate capacitor failure

• Possible explosion of electrolytic capacitors

• Regulator damage

• Board contamination

Prevention:

1. Add clear silkscreen markings on PCB

2. Include assembly notes in BOM

3. Test capacitor polarity with DMM before powering

4. Use polarized ceramic capacitors if available (more expensive)

Soldering Guidelines

Reflow soldering (recommended for production):

• Peak temperature: 260°C maximum

• Time above 220°C: 60-90 seconds

• Solder paste: SAC305 or similar lead-free

• Stencil thickness: 0.125mm (5 mil)

Hand soldering:

• Soldering iron: 350°C maximum

• Solder pin 1 (GND) first for reference

• Solder pin 2 (INPUT) second

• Apply solder to tab from component side

• Ensure good thermal contact between tab and PCB pad

Inspection Points

After assembly, inspect:

1. Electrolytic capacitor polarity: CRITICAL - verify + to GND

2. Pin solder joints: Smooth fillet, no bridges

3. Tab solder joint: Good coverage, no voids

4. Component alignment: Centered on pads

5. Thermal via filling: Solder should wick into vias

Pre-power checklist:

• [ ] C21 polarity: + terminal to GND, - terminal to -13.5V

• [ ] C22 polarity: + terminal to GND, - terminal to -12V

• [ ] Visual inspection complete

• [ ] Continuity test: GND to pin 1

• [ ] Resistance test: No shorts between -12V and GND

• [ ] Resistance test: No shorts between -13.5V and GND

Testing and Validation

Pre-Power Inspection

Before applying power, verify:

1. Capacitor polarity check:

• C21: Measure with DMM in diode mode

• C22: Measure with DMM in diode mode

• Positive terminal should be at GND potential

• Negative terminal should be connected to negative voltage rail

2. Visual inspection:

• No solder bridges

• Good solder joints on tab

• Correct IC orientation

• All components present

Initial Power-Up Test

1. Apply input voltage slowly:

• Start with -10V input (below regulation threshold)

• Gradually increase to -13.5V

• Monitor for smoke or unusual smells

• Check for thermal runaway

2. No-load test:

• Apply -13.5V to input

• Verify output voltage: -11.52V to -12.48V (-12V ±4%)

• Measure quiescent current: <10mA

• Check case temperature: Should be near ambient

Load Regulation Test

1. Connect variable load (0-1.0A)

2. Measure output voltage at different load currents:

• 0mA: Should be within -12V ±2%

• 400mA: Should be within -12V ±3%

• 800mA: Should be within -12V ±4%

3. Verify voltage variation <120mV from no-load to full-load

Thermal Test

1. Apply 0.8A load for 30 minutes

2. Measure case temperature with thermal camera

3. Verify case temperature <75°C at 25°C ambient

4. Compare to calculation: Should be ~73°C

5. Check for thermal shutdown (should not occur)

Ripple and Noise Test

1. Connect oscilloscope (AC coupling, 20MHz bandwidth limit)

2. Use short ground lead or coax probe

3. Measure output relative to GND with 0.8A load

4. Verify peak-to-peak ripple <5mVp-p (target: <1mVp-p)

5. Check for oscillation or instability

Transient Response Test

1. Use electronic load with step function (0A → 0.8A)

2. Monitor output voltage on oscilloscope

3. Verify voltage dip <200mV during load step

4. Verify recovery time <100µs

5. Check for ringing or overshoot

Troubleshooting

Application Notes

Op-Amp Power Supply Considerations

The -12V rail typically powers operational amplifiers in modular synthesizers:

Typical op-amp requirements:

• Dual supply: ±12V (some designs use ±15V)

• Current per op-amp: 2-10mA quiescent

• Peak current: Up to 30mA during output swings

• Noise sensitivity: Very high (audio applications)

Distribution recommendations:

1. Star ground topology:

• Connect all op-amp ground pins to single point

• Prevents ground loops and noise coupling

• Keep analog ground separate from digital ground

2. Local decoupling:

• Add 100nF ceramic capacitor at each op-amp power pin

• Place capacitor within 5mm of IC

• Both +12V and -12V pins need decoupling

3. Bulk capacitance:

• Add 10µF electrolytic per 4-8 op-amps

• Place in central location

• Remember polarity: + to GND, - to -12V

Audio Noise Considerations

For ultra-low-noise audio applications:

1. Additional filtering:

• Add RC filter: 10Ω + 47µF per audio section

• Creates pole at ~340Hz

• Reduces regulator noise in audio band

2. Separate analog/digital -12V:

• Use separate regulators if possible

• If sharing regulator, use isolation filters

• Prevents digital switching noise coupling to audio circuits

3. PCB layout:

• Keep -12V traces away from high-frequency signals

• Use ground plane as shield

• Route audio signals perpendicular to power traces

Dual Supply Voltage Matching

When using ±12V supplies for op-amps, voltage matching is important:

Verification:

• Measure both +12V and -12V outputs simultaneously

• Calculate: |V+12| - |V-12| should be <0.5V

• Adjust DC-DC stage if needed (modify feedback resistors)

Related Components

• Upstream: LM2596S-ADJ (U4) - Provides -13.5V input via inverting buck-boost topology

• Downstream: Protection circuit (PTC3, F3, TVS3)

• Parallel regulators: L7812CD2T-TR (U6), L7805ABD2T-TR (U7)

Common Mistakes and How to Avoid Them

Mistake 1: Reversed Electrolytic Capacitors

Problem: Installing C21 or C22 with reversed polarity Consequence: Capacitor explosion, regulator damage Prevention:

• Add large silkscreen warnings on PCB

• Test polarity with DMM before powering up

• Use checklist during assembly

Mistake 2: Confusing Positive and Negative Regulator Pinouts

Problem: Assuming pin 1 is INPUT (like 78xx series) Consequence: Wrong connections, no output Prevention:

• Clearly label pins on schematic: "Pin 1 = GND" not "Pin 1"

• Add reference designator table to schematic

• Use different symbol for negative regulators

Mistake 3: Insufficient Dropout Voltage

Problem: Input voltage not negative enough (-12V input for -12V output) Consequence: No regulation, output follows input Prevention:

• Calculate dropout: |VIN| must be >|VOUT| + 2V

• Design DC-DC stage for -13.5V or -14V

• Monitor DC-DC output voltage during testing

Mistake 4: Wrong TVS Diode Polarity

Problem: Installing TVS3 with cathode to GND (like positive rail) Consequence: No overvoltage protection, possible short circuit Prevention:

• Clearly mark TVS polarity on schematic

• For negative rail: Cathode to negative voltage, Anode to GND

• Test TVS connection with DMM (should show high resistance GND to -12V)

Mistake 5: Shared Copper Pour with Positive Rails

Problem: Connecting -12V copper to +12V copper Consequence: Direct short circuit, catastrophic failure Prevention:

• Use separate copper pours for each voltage rail

• Maintain >2mm clearance between different voltage pours

• Use DRC (Design Rule Check) to verify clearances

References

• CJ7912 Datasheet - Jiangsu Changjing Electronics

• JLCPCB Part C94173

• Negative Voltage Regulator Application Note - Texas Instruments

• LM79xx Series Datasheet - General reference for 79xx series

• Related documentation: Diagram7 - -12V Linear Regulator