Datasheet LTC3429, LTC3429B (Analog Devices) - 7

LTC3429/LTC3429B
U OPERATIO Thermal Shutdown
Diagram). However, this may adversely affect the effi- ciency and the quiescent current requirement at light An internal temperature monitor will start to reduce the loads. Typical values of C peak current limit if the die temperature exceeds 125°C. If PL range from 15pF to 220pF. the die temperature continues to rise and reaches 160°C, the part will go into thermal shutdown, all switches will be
OUTPUT DISCONNECT AND INRUSH LIMITING
turned off and the soft-start capacitor will be reset. The The LTC3429/LTC3429B are designed to allow true output part will be enabled again when the die temperature drops disconnect by eliminating body diode conduction of the by about 15°C. internal PMOS rectifier. This allows VOUT to go to zero volts during shutdown, drawing zero current from the
Burst Mode OPERATION (LTC3429 Only)
input source. It also allows for inrush current limiting at Portable devices frequently spend extended time in low start-up, minimizing surge currents seen by the input power or standby mode, only switching to high power supply. Note that to obtain the advantage of output discon- consumption when specific functions are enabled. To nect, there must not be an external Schottky diode con- improve battery life in these types of products, it is nected between the SWITCH pin and VOUT. important to maintain a high power conversion efficiency Board layout is extremely critical to minimize voltage over a wide output power range. The LTC3429 provides overshoot on the SWITCH pin due to stray inductance. automatic Burst Mode operation to increase efficiency of Keep the output filter capacitor as close as possible to the the power converter at light loads. Burst Mode operation VOUT pin and use very low ESR/ESL ceramic capacitors is initiated if the output load current falls below an inter- tied to a good ground plane. For applications with VOUT nally programmed threshold. This threshold has an in- over 4.3V, a Schottky diode is required to limit the peak verse dependence on the duty cycle of the converter and SWITCH voltage to less than 6V unless some form of also the value of the external inductor (See Typical Perfor- external snubbing is employed. This diode must also be mance Characteristics, Output Current Burst Mode Thresh- placed very close to the pins to minimize stray inductance. old vs VIN). Once Burst Mode operation is initiated, only See the Applications Information. the circuitry required to monitor the output is kept alive and the rest of the device is turned off. This is referred to
SHORT CIRCUIT PROTECTION
as the sleep state in which the IC consumes only 20µA from the output capacitor. When the output voltage droops Unlike most boost converters, the LTC3429/LTC3429B by about 1% from its nominal value, the part wakes up and allow their output to be short circuited due to the output dis- commences normal PWM operation. The output capacitor connect feature. The devices incorporate internal features recharges and causes the part to re-enter the sleep state such as current limit foldback, thermal regulation and ther- if the output load remains less than the Burst Mode mal shutdown for protection from an excessive overload threshold. The frequency of this intermittent PWM or or short circuit. In the event of a short circuit, the internal burst operation depends on the load current; that is, as the soft-start capacitor gets partially discharged. This, in turn, load current drops further below the burst threshold, the causes the maximum current limit to foldback to a smaller LTC3429 turns on less frequently. When the load current value. In addition to this, a thermal regulation circuit starts increases above the burst threshold, the LTC3429 to dial back the current limit farther if the die temperature seamlessly resumes continuous PWM operation. Thus, rises above 125°C. If the die temperature still reaches Burst Mode operation maximizes the efficiency at very 160°C, the device shuts off entirely. light loads by minimizing switching and quiescent losses. However, the output ripple typically increases to about 2%
VIN > VOUT OPERATION
peak-to-peak. Burst Mode ripple can be reduced, in some The LTC3429/LTC3429B will maintain voltage regulation circumstances, by placing a small phase-lead capacitor even if the input voltage is above the output voltage. This (CPL) between VOUT and FB pins (refer to the Block 3429fa 7