Datasheet LTC3785-1 (Analog Devices) - 9

LTC3785-1
OPERATION
The input voltage, VIN, where the four switch region begins In Burst Mode operation the maximum output current is is given by: given by: V 1.2 • V V OUT IN IN = I 1– (300ns • f) V OUT(MAX,BURST) ≈ f •L •(V ) A OUT + VIN 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 LTC3785-1. The proximately above 0.7V, switch pair C and D will alternately VCC 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 VOUT through a Schottky diode to During Burst Mode operation, the LTC3785-1 delivers provide higher gate drive in low input voltage applications. energy to the output until it is regulated and then goes The VCC regulator can also be driven with an external 5V into a sleep state where the outputs are off and the IC source directly (without a Schottky diode). is consuming only 86μA. In Burst Mode operation, the output ripple has a variable frequency component, which
TOPSIDE MOSFET DRIVER SUPPLY (V
is dependent upon load current
BST1, VBST2)
The external bootstrap capacitors connected to the V During the period where the converter is delivering en- BST1 and V ergy to the output, the inductor will reach a peak current BST2 pins supply the gate drive voltage for the top- side MOSFET switches A and D. When the top MOSFET determined by an on time, tON, and will terminate at zero switch A turns on, the switch node SW1 rises to V current for each cycle. The on time is given by: IN and the VBST2 pin rises to approximately VIN + VCC. When the t bottom MOSFET switch B turns on, the switch node SW1 ON = 2.4 V drops low and the boost capacitor is charged through the IN • f 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 V MOSFET switch C turns on, the switch node SW2 drops I IN PEAK = • t low and the boost capacitor is charged through the diode L ON connected to VCC. The boost capacitors need to store about I 100 times the gate charge required by the top MOSFET PEAK = 2.4 f •L switch A and D. In most applications a 0.1μF to 0.47μF, X5R or X7R dielectric capacitor is adequate. So the peak current is independent of VIN and inversely proportional to the f • L product optimizing the energy transfer for various applications. 37851fa 9