Datasheet LTC3808 (Analog Devices) - 10

LTC3808
U OPERATIO (Refer to Functional Diagram)
In sleep mode, much of the internal circuitry is turned off,
Short-Circuit and Current Limit Protection
reducing the quiescent current that the LTC3808 draws. The LTC3808 monitors the voltage drop ∆V The load current is supplied by the output capacitor. As the SC (between the GND and SW pins) across the external N-channel output voltage decreases, the EAMP increases the ITH MOSFET with the short-circuit current limit comparator. voltage. When the ITH voltage reaches 0.925V, the SLEEP The allowed voltage is determined by: signal goes low and the controller resumes normal opera- tion by turning on the external P-channel MOSFET on the ∆VSC(MAX) = A • 90mV next cycle of the internal oscillator. where A is a constant determined by the state of the IPRG When the controller is enabled for Burst Mode or pulse pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN skipping operation, the inductor current is not allowed to selects A = 5/3; tying IPRG to GND selects A = 2/3. reverse. Hence, the controller operates discontinuously. The inductor current limit for short-circuit protection is The reverse current comparator RICMP senses the drain- determined by ∆VSC(MAX) and the on-resistance of the to-source voltage of the bottom external N-channel external N-channel MOSFET: MOSFET. This MOSFET is turned off just before the inductor current reaches zero, preventing it from going V ∆ SC MAX ( ) negative. ISC = RDS ON ( ) In forced continuous operation, the inductor current is allowed to reverse at light loads or under large transient Once the inductor current exceeds ISC, the short current conditions. The peak inductor current is determined by the comparator will shut off the external P-channel MOSFET voltage on the I until the inductor current drops below I TH pin. The P-channel MOSFET is turned on SC. every cycle (constant frequency) regardless of the ITH pin
Output Overvoltage Protection
voltage. In this mode, the efficiency at light loads is lower than in Burst Mode operation. However, continuous mode As further protection, the overvoltage comparator (OVP) has the advantages of lower output ripple and no noise at guards against transient overshoots, as well as other more audio frequencies. serious conditions that may overvoltage the output. When the feedback voltage on the V When the SYNC/MODE pin is clocked by an external clock FB pin has risen 13.33% above the reference voltage of 0.6V, the external P-chan- source to use the phase-locked loop (see Frequency nel MOSFET is turned off and the N-channel MOSFET is Selection and Phase-Locked Loop), or is set to a DC turned on until the overvoltage is cleared. voltage between 0.4V and several hundred mV below VIN, the LTC3808 operates in PWM pulse skipping mode at
Frequency Selection and Phase-Locked Loop
light loads. In this mode, the current comparator ICMP
(PLLLPF and SYNC/MODE Pins)
may remain tripped for several cycles and force the external P-channel MOSFET to stay off for the same The selection of switching frequency is a tradeoff between number of cycles. The inductor current is not allowed to efficiency and component size. Low frequency operation reverse (discontinuous operation). This mode, like forced increases efficiency by reducing MOSFET switching losses, continuous operation, exhibits low output ripple as well as but requires larger inductance and/or capacitance to main- low audio noise and reduced RF interference as compared tain low output ripple voltage. to Burst Mode operation. However, it provides low current The switching frequency of the LTC3808’s controllers can efficiency higher than forced continuous mode, but not be selected using the PLLLPF pin. If the SYNC/MODE is nearly as high as Burst Mode operation. During start-up or not being driven by an external clock source, the PLLLPF an undervoltage condition (VFB ≤ 0.54V), the LTC3808 can be floated, tied to VIN or tied to GND to select 550kHz, operates in pulse skipping mode (no current reversal 750kHz or 300kHz, respectively. allowed), regardless of the state of the SYNC/MODE pin. 3808f 10