DESCRIPTION

The LTC3527/LTC3527-1 are dual high effi ciency, step-up DC/DC converters in a space saving 16-lead 3mm × 3mm QFN package. Battery life is maximized with a 700mV startup voltage and operation down to 500mV once started. The SHDN and PGOOD pins enable the converters to be sequenced or started together.

The LTC3527/LTC3527-1 limit inrush current during start up. Selectable 1.2MHz or 2.2MHz operation provides a choice between the highest effi ciency or smallest solution footprint. The current mode PWM design is internally compensated reducing external parts count. Burst Mode operation or fi xed frequency operation is selectable via the MODE pin. Anti-ring circuitry reduces EMI in discontinuous mode. This device also features thermal shutdown.

True output disconnect allows the output to be completely open in shutdown. The LTC3527-1 actively discharges VOUT1 or VOUT2 when its respective SHDN goes low. Quiescent current in shutdown is less than 2μA.

 

CARACTÉRISTIQUES

Dual Synchronous Step-Up DC/DC Converters

Delivers 3.3V at 200mA/100mA from one Alkaline/NiMH Cell, or 3.3V at 400mA/200mA from Two Cells

VIN Start-Up Voltage: 700mV

0.5V to 5V VIN Range After Start-Up

1.6V to 5.25V VOUT Range

Output Disconnect in Shutdown

VIN > VOUT Operation

1.2MHz or 2.2MHz Operation

Up to 94% Effi ciency

12μA Quiescent Current in Burst Mode® Operation

Inrush Current Limiting and Soft-Start

Internal Synchronous Rectifi ers

Logic-Controlled Shutdown (< 2μA)

Quick VOUT Discharge (LTC3527-1)

16-Lead, 0.75mm × 3mm × 3mm QFN Package

 

CANDIDATURES

MP3/Personal Media Players

Noise Canceling/Bluetooth Headsets

Wireless Mice

Portable Medical Instruments

 

FONCTIONNEMENT

The LTC3527/LTC3527-1 are dual 1.2MHz/2.2MHz synchronous boost converters housed in a 16-lead 3mm × 3mm QFN package. With the ability to start up and operate from inputs less than 880mV, these devices feature fixed frequency, current mode PWM control for exceptional line and load regulation. The current mode architecture with adaptive slope compensation provides excellent transient load response, requiring minimal output fi ltering. Internal soft-start and loop compensation simplifies the design process while minimizing the number of external components. Each converter has a separate input supply pin and is operated independently of the other, but they share the same oscillator thus providing in-phase switching. If different input supply voltages are used, the third VIN pin must be wired to the higher of the two supplies and each VOUT must be higher than the highest VIN. Bypass capacitors are recommended on all VIN pins.

With low RDS(ON) and low gate charge internal N-channel MOSFET switches and P-channel MOSFET synchronous rectifiers, the LTC3527/LTC3527-1 achieve high efficiency over a wide range of load current. With the MODE pin low, automatic Burst Mode operation maintains high efficiency at very light loads, reducing the quiescent current to just 12μA. If MODE is high, fi xed frequency PWM switching provides low voltage ripple on the outputs.

Low Voltage Start-Up

The LTC3527/LTC3527-1 include an independent start-up oscillator designed to start up at an input voltage of 0.7V (typical). The two converters can be started together or in either sequence of boost 1 and boost 2 with appropriate control of SHDN1 and SHDN2. Soft-start and inrush current limiting are provided to each converter independently during start-up, as well as during normal mode.

Low Noise Fixed Frequency Operation

Soft-Start: The LTC3527/LTC3527-1 contain internal circuitry to provide independent soft-start operation to each converter. The soft-start circuitry ramps the peak inductor current from zero to its peak value of 900mA (typical) for converter 1 or 500mA (typical) for converter 2 in approximately 0.5ms, allowing start-up into heavy loads. The soft-start circuitry for both converters is reset in the event of a thermal shutdown or shutdown command.

Short-Circuit Protection

The LTC3527/LTC3527-1 output disconnect feature allows an output short-circuit while maintaining a maximum internally set current limit. The converters also incorporate internal features such as current limit foldback and thermal shutdown for protection from an excessive overload or short circuit. To reduce power dissipation under shortcircuit conditions, the peak switch current limit is reduced to 500mA (typical) for converter 1 and 350mA (typical) for converter 2 when VOUT is less than 1V.