LD7120 LEADTREND | Alldatasheet

Document overview

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  • PDF pages: 8

Technical content

Features

  • Operates from 5V
  • Drives Two N-channel MOSFETs
  • Fixed 300KHz Constant Switching Frequency
  • Full 0~100% Duty-cycle
  • Programmable Over Current Protection
  • Internal Soft-start
  • Fast Transient Response
  • Under-voltage Lockout

Applications

  • Motherboard
  • Graphic Cards
  • 5V Input DC/DC Converter
  • Memory Supplies
  • Cable Modem, Set Top Box and DSL Modems
  • DSP and Core Communications Processor Supplies Typical Application FB OCSET LGATE PHASE UGATE BOOT VCC GND Vin= 5V Vout 6.19K LD7120 470pF C12 0.1uF 1uF 330uF 330uF CH551H-30 0.1uF 10K C10 8.2nF 2.15K 62 ohm C11 18nF C13 NC 0 ohm NC L1 2uH 470uF 470uF 0.1uF 1uF

Leadtrend Technology Corporation LD7120-DS-05 February 2005 Pin Configuration YY: Year code WW: Week code PP: Production code SOP-8 (TOP VIEW) 2 3 4 7 6 5 TOP MARK YYWWPP BOOT UGATE GND LGATE PHASE OCSET FB VCC

Ordering Information

Part number Package Top Mark Shipping LD7120 CS SOP-8 LD7120CS 2500 /tape & reel LD7120 PS SOP-8 (PB Free) LD7120PS 2500 /tape & reel Pin Descriptions PIN NAME FUNCTION 1 BOOT This pin provides ground reference bias voltage to the upper MOSFET driver. Connect this pin to a bootstrap circuit to pump a boot voltage to drive a N-channel MOSFET. 2 UGATE Connect UGATE pin to the PWM converte r’s upper MOSEFT gate. This pin provides the gate drive for the upper MOSFET. 3 GND Signal and power ground for the IC. All voltage levels are measured with respect to this pin. 4 LGATE Connect LGATE pin to the PWM converter’ s lower MOSEFT gate. This pin provides the gate drive for the lower MOSFET. 5 VCC Connect this pin to 5V supply voltage. This pin provides the bias for the control circuitry and the low side MOSFET driver. The voltage at this pin is monitored for Power-On Reset (POR) purpose. 6 FB This pin is the inverting input of the intern al error amplifier. Connect this pin to the output of the converter via an external resistor divider. 7 OCSET Connect a resistor from this pin to the dr ain of the upper MOSFET. This resistor, internal 20μA current source, and upper MOSFET on resistance set the over current trip point.

8 PHASE This pin is used to monitor the voltage drop across the upper MOSFET for over current

protection.

Leadtrend Technology Corporation LD7120-DS-05 February 2005 Block Diagram Control Logic 300KHz Oscillator FB OCSET GND LGATE PHASE UGATE BOOT POR & Soft-Start VCC Sample & Hold

0.8 Vref PWM

Supply Voltage Vcc -0.3~7V BOOT and UGATE to GND 15V Operating Temperature Range 0 °C to 70°C Storage Temperature Range -55°C to 125°C Junction Temperature 125°C Package Thermal Resistance, SOP-8 160°C/W Lead Temperature (Soldering, 10sec) 260°C Caution: Stresses beyond the ratings specified in “Absolute Maximum R atings” may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied.

Leadtrend Technology Corporation LD7120-DS-05 February 2005

Electrical Characteristics

(TA = +25oC unless otherwise stated, VCC=5.0V) PARAMETER CONDITIONS MIN TYP MAX UNITS Vcc Supply Current Nominal Supply UGATE and LGATE Open 2.0 mA Power On Reset Rising VCC POR Threshold 4.1 4.3 4.5 V VCC POR Threshold Hysteresis 0.4 V Oscillator Frequency Vcc=5V 250 300 340 KHz Ramp Amplitude 1.5 Vp-p Reference Reference Voltage Tolerance 2 % Nominal Reference Voltage 0.8 V Error Amplifier DC Gain 70 dB Gain Bandwidth Product 10 MHz Gate Drivers Upper Gate Source Current -1 A Upper Gate Sink Current 1 A Lower Gate Source Current -1 A Lower Gate Sink Current 2 A Protection/Disable OCSET Current Source 17.5 20 22.5 µA Disable Threshold 0.8 V

Leadtrend Technology Corporation LD7120-DS-05 February 2005 Typical Performance Characteristics LGATE UGATE Fig. 1 Dead Time LGATE UGATE Fig. 2 Dead Time VOUT VIN Fig. 3 Power ON VOUT VIN Fig. 4 Power Off IOUT VOUT Fig. 5 Load Transient VOUT Fig. 6 Load Transient IOUT

Leadtrend Technology Corporation LD7120-DS-05 February 2005 UGATE Fig. 7 Bootstrap Waveform LGATE Phase VOUT Fig. 8 OCP Waveform IOUT

Application Information

The LD7120 automatically initializes upon receipt of power. The Power On Reset (POR) function continually monitors the bias voltage at the VCC pin. The POR function initiates the Over Current Protection (OCP) sampling and hold operation after the supply voltage exceeds its POR threshold. Upon completion of the OCP sampling and hold operation, the POR function initiates the Soft Start operation. Over Current Protection The over current function cycles the soft start function in a hiccup mode to provide fault protection. A resistor R OCSET from OCSET pin to the drain of high side MOSFET programs the over current trip level. The resistor, an internal 20μA current source IOCSET and the MOSFET on resistance set the over current trip point IPEAK according to the following formula: DS(ON) OCSETOCSETPEAK R RII ×= To avoid over current tripping in the normal operating load range, find the ROCSET resistor form the equation above with: 1. The maximum R DS(ON) at the highest junction temperature. 2. The minimum I OCSET from the specification table. 3. Determine I PEAK for IPEAK > IOUT(MAX)+ 2 Where ∆I is the output inductor ripple current. Pulling and holding this pin below 0.8V with an open drain device will shut down the IC. Releasing OCSET will initiate soft start on the controller. Soft Start The POR function initiates the soft start function after the over current set point has been sampled. The built in soft start function is to prevent inrush current and output voltage overshoot during power on. An internal digital counter controls the soft start voltage. It clamps the ramping of reference voltage at the input of error amplifier and the pulse width of the output driver slowly. The typical soft start duration is 6.8mS. Bootstrap Operation In a single power supply system, an external bootstrap circuit, as Fig.9, powers the UGATE driver of LD7120. The boot capacitor, Cboot, generates a floating reference at the PHASE pin. Typical 0.1 µF Cboot is enough for most of MOSFETs used with the LD7120. The voltage drop between BOOT and PHASE is refreshed to a voltage of Vcc - diode drop while the low side MOSFET turning on.

Leadtrend Technology Corporation LD7120-DS-05 February 2005 LGATE PHASE UGATE BOOT VCC Vin LD7120 Vout Vcc Cboot 0.1uF 1uF F i g . 9 Layout Consideration Layout is very important in high switching convert design. There are two sets of critical components in DC/DC converter using LD7120.The switching components are the most critical because they switch large amount of energy, and therefore tend to generate large amount of noise. Next are the small signal components, which connect to sensitive nodes, or those supplying critical bypass current and signal coupling. Care should be taken to separate these two sets of components and their associated grounds. Below are some guidelines for PCB layout while using LD7120. 1. Keep the bypass capacitor 1 µF very close to IC. 2. Keep output voltage feed back network, FB pin and OCSET pin related components (small signal components) very close to IC. 3. Signal ground plane of FB and OCSET pin (small signal components) should be connected to the power ground plane with a via or only one point to minimize the effect of power ground currents. 4. Switching node such as BOOT, UGATE, PHASE and LGATE should be kept as small as possible and routed away from FB and OCSET pin. 5. The PCB traces carrying discontinuous currents and any high current path should be made as short and wide as possible. 6. If possible, a multi-layer PCB is recommended. Please refer to the EV kit of LD7120 for a PCB layout example.

Leadtrend Technology Corporation LD7120-DS-05 February 2005

Package Information

Package: SOP-8 D C I F J A B H M Dimensions in Millimeters Dimensions in Inch Symbols MIN MAX MIN MAX A 4.801 5.004 0.189 0.197 B 3.810 3.988 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.508 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.178 0.229 0.007 0.009 I 0.102 0.254 0.004 0.010 J 5.791 6.198 0.228 0.244 M 0.406 1.270 0.016 0.050 θ 0 8° 0 8° Important Notice Leadtrend Technology Corp. reserves the right to make changes or corrections to its products at any time without notice. Custom ers should verify the datasheets are current and complete before placing order.