LTC3859AL datasheet(16/42 Pages) LINER

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LTC7813

7813f

For more information www.linear.com/LTC7813

operaTion

The switching frequency of the LTC7813’s controllers can

be selected using the FREQ pin.

If the PLLIN/MODE pin is not being driven by an external

clock source, the FREQ pin can be tied to GND, tied to

INTVCCorprogrammedthroughanexternalresistor.Tying

FREQ to GND selects 350kHz while tying FREQ to INTVCC

selects 535kHz. Placing a resistor between FREQ and GND

allows the frequency to be programmed between 50kHz

and 900kHz, as shown in Figure 10.

A phase-locked loop (PLL) is available on the LTC7813

to synchronize the internal oscillator to an external clock

source that is connected to the PLLIN/MODE pin. The

LTC7813’s phase detector adjusts the voltage (through

an internal lowpass filter) of the VCO input to align the

turn-on of TG1 and BG2 to the rising edge of the syn-

chronizing signal.

The VCO input voltage is prebiased to the operating fre-

quency set by the FREQ pin before the external clock is

applied. If prebiased near the external clock frequency,

the PLL loop only needs to make slight changes to the

VCO input in order to synchronize the rising edge of the

external clock’s to the rising edge of TG1 and BG2. The

ability to prebias the loop filter allows the PLL to lock-in

rapidly without deviating far from the desired frequency.

The typical capture range of the LTC7813’s phase-locked

loop is from approximately 55kHz to 1MHz, with a guaran-

tee to be between 75kHz and 850kHz. In other words, the

LTC7813’s PLL is guaranteed to lock to an external clock

source whose frequency is between 75kHz and 850kHz.

The typical input clock thresholds on the PLLIN/MODE

pin are 1.6V (rising) and 1.1V (falling). It is recommended

that the external clock source swings from ground (0V)

to at least 2.5V.

Boost Controller Operation When VIN2 > VOUT2

When the input voltage to the boost channel rises above

its regulated VOUT2 voltage, the controller can behave

differently depending on the mode, inductor current and

VIN2 voltage. In forced continuous mode, the loop works

to keep the top MOSFET on continuously once VIN2 rises

above VOUT2. An internal charge pump delivers current to

the boost capacitor from the BOOST2 pin to maintain a

sufficiently high TG2 voltage. Because the LTC7813 uses

internal switches and does not require external bootstrap

diodes, the charge pump only has to overcome small

leakage currents (board leakage, etc.).

In pulse-skipping mode, if VIN is between 0% and 10%

above the regulated VOUT2 voltage, TG2 turns on if the

inductor current rises above approximately 3% of the

programmed ILIM current. If the part is programmed in

Burst Mode operation under this same VIN2 window, then

TG2 turns on at the same threshold current as long as

the chip is awake (the buck channel is awake and switch-

ing). If the buck channel is asleep or shut down in this

VIN2 window, then TG2 will remain off regardless of the

inductor current.

If VIN rises more than 10% above the regulated VOUT

voltage in any mode, the controller turns on TG2 regard-

less of the inductor current. In Burst Mode operation,

however, the internal charge pump turns off if the entire

chip is asleep (if the buck channel is also asleep or shut

down). With the charge pump off, there would be nothing

to prevent the boost capacitor from discharging, result-

ing in an insufficient TG2 voltage needed to keep the top

MOSFET completely on. The charge pump turns back on

when the chip wakes up, and it remains on as long as the

buck channel is actively switching.

Boost Controller at Low SENSE Pin Common Voltage

The current comparator of the boost controller is powered

directly from the SENSE2+ pin and can operate to voltages

as low as 2.2V. Since this is lower than the VBIAS UVLO of

the chip, VBIAS can be connected to the output of the boost

controller, as illustrated in the typical application circuit in

Figure 12. This allows the boost controller to handle input

voltage transients down to 2.2V while maintaining output

voltage regulation. If SENSE2+ falls below 2.0V, then

switching stops and SS2 is pulled low. If SENSE2+ rises

back above 2.2V, the SS2 pin will be released, initiating a

new soft-start sequence.

(Refer to the Functional Diagrams)

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