Datasheet LTC3535 (Analog Devices) - 10

LTC3535
OPERATION (Refer to Block Diagram)
the test mode being engaged. Refer to Figure 1 for two independent of input or output voltage, unless VOUT falls possible implementations. below 0.7V, in which case the current limit is cut in half.
Zero Current Comparator
LTC3535 LTC3535 VIN The zero current comparator monitors the inductor cur- 4M 4M rent to the output and shuts off the synchronous rectifier ±30% ±30% SHDN SHDN when this current reduces to approximately 30mA. This 3535 F01 R V prevents the inductor current from reversing in polarity, CNTRL ZETEX improving efficiency at light loads. 1M ZC2811E
Synchronous Rectifier
R > (VCNTRL/VIN + 0.4) – 1)MΩ 1M To control inrush current and to prevent the inductor VCNTRL current from running away when VOUT is close to VIN, the
Figure 1. Recommended Shutdown Circuits When
P-channel MOSFET synchronous rectifier is only enabled
Driving SHDN Above VIN
when VOUT > (VIN + 0.24V).
Anti-Ringing Control Error Amplifier
The anti-ring circuit connects a resistor across the in- The positive input of the transconductance error amplifier ductor to prevent high frequency ringing on the SW pin is internally connected to the 1.195V reference and the during discontinuous current mode operation. Although negative input is connected to FB. Clamps limit the mini- the ringing of the resonant circuit formed by L and C mum and maximum error amp output voltage for improved SW (capacitance on SW pin) is low energy, it can cause EMI large-signal transient response. Power converter control radiation. loop compensation is provided internally. An external resistive voltage divider from VOUT to ground programs
Output Disconnect
the output voltage via FB from 1.5V to 5.25V. The LTC3535 is designed to allow true output disconnect  R2 V 1 195 . V  • 1 by eliminating body diode conduction of the internal OUT = +  R1 P-channel MOSFET rectifier. This allows for VOUT to go to zero volts during shutdown, drawing no current from
Current Sensing
the input source. It also allows for inrush current limiting at turn-on, minimizing surge currents seen by the input Lossless current sensing converts the peak current signal of supply. Note that to obtain the advantages of output dis- the N-channel MOSFET switch into a voltage that is summed connect, there must not be an external Schottky diode with the internal slope compensation. The summed signal connected between SW and VOUT. The output disconnect is compared to the error amplifier output to provide a peak feature also allows VOUT to be pulled high, without any current control command for the PWM. reverse current into a battery connected to VIN.
Current Limit Thermal Shutdown
The current limit comparator shuts off the N-channel If the die temperature exceeds 160°C, the LTC3535 will MOSFET switch once its threshold is reached. The cur- go into thermal shutdown. All switches will be off and rent limit comparator delay to output is typically 60ns. the soft-start capacitor will be discharged. The device Peak switch current is limited to approximately 750mA, will be enabled again when the die temperature drops by about 15°C. 3535fa 10 Document Outline Features Description Applcations Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts