Datasheet LTC3785 (Analog Devices) - 9

LTC3785 operaTion the buck-boost range, approximately 0.7V, the AC switch So the peak current is independent of VIN and inversely pair completely phase out the BD pair, and the boost phase proportional to the f • L product optimizing the energy begins at duty cycle, D4(SW). transfer for various applications. The input voltage, VIN, where the four switch region begins In Burst Mode operation the maximum output current is is given by: given by: 1.2 • V V I IN V OUT OUT(MAX,BURST) ≈ f •L •(V ) A IN = OUT + VIN 1– (300ns• f) V the point at which the four switch region ends is given Burst Mode operation is user-controlled by driving the by: MODE pin high to enable and low to disable. VIN = VOUT(1 – D) = VOUT(1 – 300ns • f) V
VCC REGULATOR Boost Region (VIN < VOUT)
An internal P-channel low dropout regulator produces Switch A is always on and switch B is always off during 4.35V at the VCC pin from the VIN supply pin. VCC powers boost mode. When the error amp output voltage, VC, is ap- the drivers and internal circuitry of the LTC3785. The VCC proximately above 0.7V, switch pair C and D will alternately pin regulator can supply a peak current of 100mA and switch to provide a boosted output voltage. This operation must be bypassed to ground with a minimum of 4.7µF is typical to a synchronous boost regulator. The maximum placed directly adjacent to the VCC and GND pins. Good duty cycle of the converter is limited to 90% typical. bypassing is necessary to supply the high transient cur- rent required by the MOSFET gate drivers and to prevent
Burst Mode OPERATION
interaction between channels. If desired, the VCC regulator can be connected to V During Burst Mode operation, the LTC3785 delivers energy OUT through a Schottky diode to provide higher gate drive in low input voltage applications. to the output until it is regulated and then goes into a sleep The V state where the outputs are off and the IC is consuming CC regulator can also be driven with an external 5V source directly (without a Schottky diode). only 86µA. In Burst Mode operation, the output ripple has a variable frequency component, which is dependent upon load current.
TOPSIDE MOSFET DRIVER SUPPLY (VBST1, VBST2)
During the period where the converter is delivering en- The external bootstrap capacitors connected to the VBST1 ergy to the output, the inductor will reach a peak current and VBST2 pins supply the gate drive voltage for the top- determined by an on time, t side MOSFET switches A and D. When the top MOSFET ON, and will terminate at zero current for each cycle. The on time is given by: switch A turns on, the switch node SW1 rises to VIN and the V 2.4 BST2 pin rises to approximately VIN + VCC. When the t bottom MOSFET switch B turns on, the switch node SW1 ON = VIN • f drops low and the boost capacitor is charged through the diode connected to V where f is the oscillator frequency. CC. When the top MOSFET switch D turns on, the switch node SW2 rises to VOUT and the VBST2 The peak current is given by: pin rises to approximately VOUT + VCC. When the bottom MOSFET switch C turns on, the switch node SW2 drops V I IN low and the boost capacitor is charged through the diode PEAK = • t L ON connected to VCC. The boost capacitors need to store about 2.4 I 100 times the gate charge required by the top MOSFET PEAK = f •L 3785fc Document Outline Features Applications Description 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