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LTC1649

3.3V Input High Power

Step-Down Switching

Regulator Controller

■

High Power 3.3V to 1.xV-2.xV Switching Regulator

Controller: Up to 20A Output

■

All N-Channel External MOSFETs

■

Provides 5V MOSFET Gate Drive with 3.3V Input

■

Excellent Output Regulation: ±1% Over Line, Load

and Temperature Variations

■

Constant Frequency Operation Minimizes

Inductor Size

■

High Efficiency: Over 90% Possible

■

No Low-Value Sense Resistor Needed

■

Available in 16-Lead SO Package

■

3.3V Input Power Supply for Low Voltage

Microprocessors and Logic

■

Low Input Voltage Power Supplies

■

High Power, Low Voltage Regulators

■

Local Regulation for Multiple Voltage Distributed

Power Systems

, LTC and LT are registered trademarks of Linear Technology Corporation.

The LTC

1649 is a high power, high efficiency switching

regulator controller optimized for use with very low supply

voltages. It operates from 2.7V to 5V input, and provides

a regulated output voltage from 1.26V to 2.5V at up to 20A

load current. A typical 3.3V to 2.5V application features

efficiency above 90% from 1A to 10A load. The LTC1649

uses a pair of standard 5V logic-level N-channel external

MOSFETs, eliminating the need for expensive P-channel

or super-low-threshold devices.

The LTC1649 shares its internal switching architecture

with the LTC1430, and features the same ±1% line, load

and temperature regulation characteristics. Current limit

is user-adjustable without requiring an external low-value

sense resistor. The LTC1649 uses a 200kHz switching

frequency and voltage mode control, minimizing external

component count and size. Shutdown mode drops the

quiescent current to below 10µA.

The LTC1649 is available in the 16-pin narrow SO package.

OUT

2.5V

@15A

MAX

SHDN

1µF

3.3V

LTC1649

VCC2

VCC1

COMP

SS C

–

GND CP

OUT

1µF

10µF

MBR0530

0.1µF

0.01µF

IRF7801 = INTERNATIONAL RECTIFIER

MBR0530 = MOTOROLA

*12TS-1R2HL = PANASONIC

7.5k

EXT

1.2µH

220pF

10µF

MBR0530

IMAX

50k

22Ω 1k

IRF7801

Q1, Q2

IRF7801

TWO IN

PARALLEL

OUT

4400µF

3300µF

SHDN

12.7k

12.4k

1649 TA01

0.33µF

3.3V to 2.5V, 15A Converter LTC1649 Efficiency

LOAD CURRENT (A)

EFFICIENCY (%)

1649 TA02

0.1 1 10

100

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

Summary of content (16 pages)

PAGE 1
LTC1649 3.3V Input High Power Step-Down Switching Regulator Controller U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1649 is a high power, high efficiency switching regulator controller optimized for use with very low supply voltages. It operates from 2.7V to 5V input, and provides a regulated output voltage from 1.26V to 2.5V at up to 20A load current. A typical 3.3V to 2.5V application features efficiency above 90% from 1A to 10A load.
PAGE 2
LTC1649 W W U W ABSOLUTE MAXIMUM RATINGS U W U PACKAGE/ORDER INFORMATION (Note 1) Supply Voltage VIN ........................................................................................... 6V VCC ........................................................................................... 9V PVCC1, 2 ................................................................................ 13V Input Voltage IFB ....................................................................... – 0.3V to 18V C +, C – ...
PAGE 3
LTC1649 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a part may be impaired. Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 3: Maximum Duty Cycle limitations will limit the output voltage obtainable at very low supply voltages.
PAGE 4
LTC1649 U U U PIN FUNCTIONS G1 (Pin 1): Driver Output 1. Connect this pin to the gate of the upper N-channel MOSFET, Q1. This output will swing from PVCC1 to GND. G1 will always be low when G2 is high. In shutdown, G1 and G2 go low. PVCC1 (Pin 2): Power VCC for Driver 1. This is the power supply input for G1. G1 will swing from PVCC1 to GND. PVCC1 must be connected to a potential of at least VIN + VGS(ON)(Q1).
PAGE 5
LTC1649 W BLOCK DIAGRA C+ C– CHARGE PUMP VIN CPOUT DELAY SHDN INTERNAL SHUTDOWN 50µs PVCC1 G1 PWM COMP PVCC2 G2 VCC 12µA SS ILIM – FB + + MIN MAX – IMAX IFB 12µA FB + 40mV 40mV + + 1649 BD 1.26V TEST CIRCUIT VIN 3.3V RIMAX 50k Q1, Q2 IRF7801 TWO IN PARALLEL 1µF MBR0530 PVCC1 CIN 3300µF LEXT 1.2µH VOUT 2.5V G1 22Ω 1k PVCC2 IFB Q3 IRF7801 G2 VCC LTC1649 FB IMAX SHDN + SHDN + R1 12.4k + VIN + 10µF COMP 1µF C– SS RC 7.5k C GND COUT 4400µF R2 12.
PAGE 6
LTC1649 U W U U APPLICATIONS INFORMATION OVERVIEW The LTC1649 is a voltage feedback PWM switching regulator controller (see Block Diagram) designed for use in high power, low input voltage step-down (buck) converters. It includes an onboard PWM generator, a precision reference trimmed to ±0.5%, two high power MOSFET gate drivers and all necessary feedback and control circuitry to form a complete switching regulator circuit.
PAGE 7
LTC1649 U W U U APPLICATIONS INFORMATION MOSFET Gate Drive Synchronous Operation The LTC1649 is designed to operate from supplies as low as 2.7V while using standard 5V logic-level N-channel external MOSFETs. This poses somewhat of a challenge— from as little as 2.7V, the LTC1649 must provide a 0V to 5V signal to the lower MOSFET, Q2, while the upper MOSFET, Q1, requires a gate drive signal that swings from 0V to (VIN + 5V). The LTC1649 addresses this situation with two specialized circuits.
PAGE 8
LTC1649 U W U U APPLICATIONS INFORMATION EXTERNAL COMPONENT SELECTION Power MOSFETs Two N-channel power MOSFETs are required for most LTC1649 circuits. These should be selected primarily by on-resistance considerations; thermal dissipation is often a secondary concern in high efficiency designs. The LTC1649 is designed to be used with 5V logic-level MOSFETs; “standard” threshold MOSFETs with RDS(ON) specified at 10V only will not provide satisfactory performance.
PAGE 9
LTC1649 U W U U APPLICATIONS INFORMATION ages and expected loads may require different values. Once the required value is known, the inductor core type can be chosen based on peak current and efficiency requirements. Peak current in the inductor will be equal to the maximum output load current added to half the peakto- peak inductor ripple current. Ripple current is set by the inductor value, the input and output voltage and the operating frequency.
PAGE 10
LTC1649 U U W U APPLICATIONS INFORMATION tors that work well in LTC1649 applications. A common way to lower ESR and raise ripple current capability is to parallel several capacitors. A typical LTC1649 application might require an input capacitor with a 5A ripple current capacity and 2% output shift with a 10A output load step, which requires a 0.005Ω output capacitor ESR. Sanyo OSCON part number 10SA220M (220µF/10V) capacitors feature 2.3A allowable ripple current at 85°C and 0.
PAGE 11
LTC1649 U W U U APPLICATIONS INFORMATION The CPOUT pin can typically supply 50mA at 5V, adequate to power the VCC and PVCC pins. This supply can also be used to power external circuitry, but any additional current drawn from CPOUT subtracts from the current available to drive the external MOSFETs. Circuits with small external MOSFETs can draw as much as 20mA or 30mA from CPOUT without hindering performance.
PAGE 12
LTC1649 U U W U APPLICATIONS INFORMATION The LTC1649 detects the output current by watching the voltage at IFB while Q1 is ON. The ILIM amplifier compares this voltage to the voltage at IMAX (Figure 7). In the ON state, Q1 has a known resistance; by calculating backwards, the voltage generated at IFB by the maximum output current in Q1 can be determined. As IFB falls below IMAX, ILIM will begin to sink current from the soft start pin, causing the voltage at SS to fall.
PAGE 13
LTC1649 U W U U APPLICATIONS INFORMATION resume normal operation. The CPOUT pin remains regulated at 5V in shutdown, and can be used as a keep-alive supply for external circuitry if desired. Note that any current drawn from the CPOUT pin adds to the quiescent current in shutdown, and subtracts from the current available to drive the external MOSFETs if the load remains connected while the LTC1649 is active.
PAGE 14
LTC1649 U U W U APPLICATIONS INFORMATION and output capacitors, the source of Q2, the LTC1649 GND pin, the output return and the input supply return all clustered at one point. Figure 9 is a modified schematic showing the common connections in a proper layout. Note that at 10A current levels or above, current density in the PC board itself can become a concern; traces carrying high currents should be as wide as possible.
PAGE 15
LTC1649 U PACKAGE DESCRIPTIO S Package 16-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) 0.386 – 0.394* (9.804 – 10.008) 16 15 14 13 12 11 10 9 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 2 3 4 5 6 0.053 – 0.069 (1.346 – 1.752) 0.008 – 0.010 (0.203 – 0.254) 0.014 – 0.019 (0.355 – 0.483) TYP 8 0.004 – 0.010 (0.101 – 0.254) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 7 0.050 (1.
PAGE 16
LTC1649 U TYPICAL APPLICATIO SSTL Logic Termination Supply VIN 3.3V MBR0530 + Q1 1/2 Si9802 1µF CERAMIC 1500µF LEXT 1.5µH PVCC1 G1 PVCC2 IFB VOUT = 0.45VIN AT ±5A = 1.48V AT VIN = 3.3V 22Ω NC VCC G2 LTC1649 IMAX FB SHDN SHDN 10µF CERAMIC COMP R1 18.2k + 1500µF R2 15k VIN C+ 1µF CERAMIC C– SS 7.5k Q2 1/2 Si9802 GND CPOUT 0.01µF 220pF + 0.1µF MBR0530 10µF 50pF 2200pF 4.7k 10k 0.