Datasheet

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January 2012 Doc ID 18134 Rev 2 1/32

L6738A

Single-phase PWM controller

with light-load efficiency optimization

Features

■ Flexible power supply from 5 V to 12 V bus

■ Power conversion input as low as 1.5 V

■ Light-load efficiency optimization

■ Embedded bootstrap diode

■ VIN detector

■ 0.8 V internal reference

■ 0.5% output voltage accuracy

■ Remote GND recovery

■ High-current integrated drivers

■ Sensorless and programmable precise-OC

sense across Inductor DCR

■ OV protection

■ Programmable oscillator up to 600 kHz

■ LS-less to manage pre-bias startup

■ Adjustable output voltage

■ Disable function

■ Internal soft-start

■ VFQFPN 16 3x3 mm package

Applications

■ Memory and termination supply

■ Subsystem power supply (MCH, IOCH, PCI)

■ CPU and DSP power supply

■ Distributed power supply

■ General DC-DC converter

Description

The L6738A is a single-phase step-down

controller with integrated high-current drivers that

provides complete control logic and protection to

realize a DC-DC converter.

The device flexibility allows to manage

conversions with power input V

as low as 1.5V

and the device supply voltage ranging from 5 V to

12 V bus.

The L6738A features a proprietary algorithm that

allows Light-Load efficiency optimization,

boosting efficiency without compromising the

output voltage ripple.

The integrated 0.8 V reference allows generation

of output voltages with ±0.5% accuracy over line

and temperature variations.

The oscillator is programmable up to 600 kHz.

The L6738A provides a programmable

overcurrent protection and overvoltage protection.

The current information is monitored across the

inductor DCR.

The L6738A is available in a VFQFPN 16 3x3 mm

package.

Table 1. Device summary

Order code Package Packing

L6738A VFQFPN16 Tube

L6738ATR VFQFPN16 Tape and reel

VFQFPN16

www.st.com

Summary of content (32 pages)

PAGE 1
L6738A Single-phase PWM controller with light-load efficiency optimization Features ■ Flexible power supply from 5 V to 12 V bus ■ Power conversion input as low as 1.5 V ■ Light-load efficiency optimization ■ Embedded bootstrap diode ■ VIN detector ■ 0.8 V internal reference ■ 0.
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Contents L6738A Contents 1 2 3 Typical application circuit and block diagram . . . . . . . . . . . . . . . . . . . . 5 1.1 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Pin description and connection diagrams . . . . . . . . . . . . . . . . . . . . . . . 8 2.1 Pin descriptions . . . . . . . . . . . . . . . . . . . . . . .
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L6738A Contents 10.3 Input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 11 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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List of tables L6738A List of tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. 4/32 Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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L6738A List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Typical application circuit (fast protections) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Typical application circuit and block diagram L6738A 1 Typical application circuit and block diagram 1.1 Application circuit Figure 1. Typical application circuit (fast protections) VCC = 5V to 12V CDEC ROSC PGOOD GND OSC VIN = 1.
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L6738A Typical application circuit and block diagram Figure 2. Typical application circuit VCC = 5V to 12V CDEC ROSC PGOOD GND OSC VIN = 1.
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Typical application circuit and block diagram Block diagram Figure 3. Block diagram OVP VCC PGOOD 1.2 L6738A UVP OVER CURRENT CONTROL LOGIC, MONITOR, PROTECTIONS & EFFICIENCY OPTIMIZATION OCP 2.2 CSN CSP BOOT PROGRAMMABLE OSCILLATOR EN OVP PWM +25% 10k FBG FB COMP VSEN 8/32 LGATE LS UVP + - Doc ID 18134 Rev 2 0.
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L6738A 2 Pin description and connection diagrams Pin description and connection diagrams Pin connection (top view) GND VCC OSC CSP Figure 4. 16 15 14 13 VCCDR LGATE PHASE BOOT 1 12 2 11 3 L6738A 10 9 4 5 6 7 CSN FBG VSEN FB 8 COMP EN PGOOD UGATE 2.1 Pin descriptions Table 2. Pin description Pin# Name Function 1 VCCDR Low side driver section power supply. Operative voltage is 5 V to 12 V bus. Filter with 1µF MLCC to GND. 2 LGATE Low-side driver output.
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Pin description and connection diagrams Table 2. L6738A Pin description (continued) Pin# Name 8 COMP 9 FB 10 VSEN 11 FBG Remote ground sense. Connect to the negative side of the load for remote sensing. 12 CSN Current sense negative input. Connect to the output-side of the main inductor. Filter with 100 nF (Typ.) to GND. 13 CSP Current sense positive input. Connect through an R-C filter to the phase-side of the main inductor. 14 OSC OSC: Internally set to 1.
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L6738A Electrical specifications 3 Electrical specifications 3.1 Absolute maximum ratings Table 4. Absolute maximum ratings Symbol Value Unit to GND -0.3 to 15 V VBOOT, VUGATE to GND to PHASE to GND, VCCDR = 12 V, t < 200 nsec. -0.3 to 41 15 45 V VPHASE to GND to GND, VCCDR = 12 V, t < 200 nsec. -5 to 26 -8 to 30 V VLGATE to GND -0.3 to VCCDR + 0.3 V to GND, VCC < 7 V to GND, VCC > 7 V -0.3 to VCC + 0.3 -0.3 to 7 V to GND(1) -0.3 to VCC - 1.5 V -0.3 to 3.
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Electrical specifications Table 5. Symbol L6738A Electrical characteristics (continued) Parameter ∆VOSC PWM ramp amplitude d Duty cycle Test conditions Min. Typ. Max. 2 0 Input logic high EN rising Input logic low EN falling Unit V 100 0.9 % V EN 0.5 V 0.5 % Reference and error amplifier Output voltage accuracy A0 DC gain Vout to FBG -0.
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L6738A 4 Device description and operation Device description and operation The L6738A is a single-phase PWM controller with embedded high-current drivers that provides complete control logic and protections to realize a general DC-DC step-down converter. Designed to drive N-channel MOSFETs in a synchronous buck topology, with its high level of integration, this 16-pin device allows a reduction of cost and size of the power supply solution and also provides real-time PGOOD in a compact VFQFPN16 3x3 mm.
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Soft-start 5 L6738A Soft-start The L6738A implements a soft-start to smoothly charge the output filter avoiding high inrush currents to be required to the input power supply. During this phase, the device increases the internal reference from zero up to 0.8 V in closed loop regulation. The softstart is implemented only when VCC and VCCDR are above their own UVLO threshold and the EN pin is set free.
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L6738A Soft-start Figure 5. LS-less startup (left) vs.
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Output voltage setting and protections 6 L6738A Output voltage setting and protections The L6738A is capable of precisely regulating an output voltage as low as 0.8 V. In fact, the device comes with a fixed 0.8 V internal reference that guarantees the output regulated voltage to be within ±0.5% tolerance over line and temperature variations (excluding output resistor divider tolerance, when present). Output voltage higher than 0.
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L6738A Output voltage setting and protections For proper operations, VCC needs to be at least 1.5 V higher than the programmed output voltage. Table 6. L6738A Protection at a glance. L6738A Overvoltage (OV) Comments VSEN = +12.5% above reference. Action: IC Latch; LS=ON until VSEN = 50% of Vref; PGOOD = GND. Action (EN=0): IC Latch; LS=ON; reset by EN rising edge (PreOVP). VSEN = -25% below reference. Undervoltage (UV) Action: IC Latch; HiZ; PGOOD = GND. Action (SS): SS reset (VIN Detection).
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Output voltage setting and protections 6.2 L6738A Overcurrent threshold setting The L6738A detects OC when the difference between CSP and CSN is equal to 20 mV (typ). By properly designing the current reading network, it is possible to program the OC threshold as desired (See Figure 6).
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L6738A 7 Main oscillator Main oscillator The controller embeds a programmable oscillator. The internal oscillator generates the sawtooth waveform for the PWM charging with a constant current and resetting an internal capacitor. The switching frequency, FSW, is internally fixed at 200 kHz.
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High Current embedded drivers 8 L6738A High Current embedded drivers The L6738A provides high-current driving control. The driver for the high-side MOSFET uses the BOOT pin for supply and the PHASE pin for return. The driver for the low-side MOSFET uses the VCCDR pin for supply and the GND pin for return.
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L6738A High Current embedded drivers Two main terms contribute in the device power dissipation: bias power and drivers' power. ● Device Power (PDC) depends on the static consumption of the device through the supply pins and it is simply quantifiable as follows: P DC = VCC ⋅ ICC + V VCCDR ⋅ I VCCDR ● Drivers' power is the power needed by the driver to continuously switch ON and OFF the external MOSFETs; it is a function of the switching frequency and total gate charge of the selected MOSFETs.
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Application details L6738A 9 Application details 9.1 Compensation network The control loop shown in Figure 9 is a voltage mode control loop. The output voltage is regulated to the internal reference (when present, an offset resistor between FB node and GND can be neglected in control loop calculation). Error amplifier output is compared to the oscillator sawtooth waveform to provide a PWM signal to the driver section. The PWM signal is then transferred to the switching node with VIN amplitude.
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L6738A Application details Figure 10. Example of type III compensation.
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Application details L6738A a) Set the gain RF/RFB in order to obtain the desired closed loop regulator bandwidth according to the approximated formula (suggested values for RFB are in the range of some kΩ): F0dB ∆V OSC RF ---------= ------------ ⋅ ------------------FLC V IN R FB b) Place FZ1 below FLC (typically 0.
PAGE 25
L6738A Application details Connect output bulk capacitors (COUT) as near as possible to the load, minimizing parasitic inductance and resistance associated to the copper trace, also adding extra decoupling capacitors along the way to the load when this results in being far from the bulk capacitors bank. Remote Sense Connection must be routed as parallel nets from the FBG/VSEN pins to the load in order to avoid the pick-up of any common mode noise.
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Application information L6738A 10 Application information 10.1 Inductor design The inductance value is defined by a compromise between the dynamic response time, the efficiency, the cost, and the size. The inductor must be calculated to maintain the ripple current (∆IL) between 20% and 30% of the maximum output current (typ).
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L6738A 10.2 Application information Output capacitor(s) The output capacitors are basic components to define the ripple voltage across the output and for the fast transient response of the power supply. They depend on the output voltage ripple requirements, as well as any output voltage deviation requirement during a load transient.
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Application information L6738A therefore in higher ripple voltages. For these reasons, a mix between electrolytic and MLCC capacitors is suggested to minimize ripple as well as reduce voltage deviation in dynamic mode. 10.
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L6738A 11 Package mechanical data Package mechanical data In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK® packages, depending on their level of environmental compliance. ECOPACK specifications, grade definitions, and product status are available at www.st.com. ECOPACK is an ST registered trademark.
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Package mechanical data L6738A Figure 14. VFQFPN16 mechanical data and package dimensions mm mils DIM. MIN. TYP. MAX. MIN. TYP. MAX. 0.80 31.49 0.90 1.00 35.43 39.37 A1 0.02 0.05 0.78 1.96 A2 0.65 1.00 25.59 39.37 A A3 7.87 b 0.18 0.25 0.30 7.08 9.84 11.81 D 2.85 3.00 3.15 112.2 118.1 124.0 D1 1.50 59.05 D2 1.60 62.99 E 2.85 3.00 3.15 112.2 118.1 E1 1.50 59.05 E2 1.60 62.99 124.0 e 0.45 0.50 0.55 17.71 19.68 21.65 L 0.30 0.40 0.50 11.
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L6738A 12 Revision history Revision history Table 7. Document revision history Date Revision Changes 03-Nov-2010 1 Initial release.
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L6738A Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale.