LTC3851AEGN#PBF Datasheet LTC3851AIUD#PBF, LTC3851AIGN#PBF, LTC3851AMPMSE#PBF | P10-P12

LTC3851A

3851afa

operaTion

Main Control Loop

The LTC3851A is a constant frequency, current mode

step-down controller. During normal operation, the top

MOSFET is turned on when the clock sets the RS latch,

and is turned off when the main current comparator, I

CMP

resets the RS latch. The peak inductor current at which

CMP

resets the RS latch is controlled by the voltage on

the I

pin, which is the output of the error amplifier, EA.

The V

pin receives the voltage feedback signal, which is

compared to the internal reference voltage by the EA. When

the load current increases, it causes a slight decrease in

relative to the 0.8V reference, which in turn causes the

voltage to increase until the average inductor current

matches the new load current. After the top MOSFET has

turned off, the bottom MOSFET is turned on until either

the inductor current starts to reverse, as indicated by the

reverse current comparator, I

REV

, or the beginning of the

next cycle.

INTV

Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTV

pin. An

internal 5V low dropout linear regulator supplies INTV

power from V

The top MOSFET driver is biased from the floating boot-

strap capacitor, C

, which normally recharges during each

off cycle through an external diode when the top MOSFET

turns off. If the input voltage, V

, decreases to a voltage

close to V

OUT

, the loop may enter dropout and attempt

to turn on the top MOSFET continuously. The dropout

detec tor detects this and forces the top MOSFET off for

about 1/10 of the clock period every tenth cycle to allow

to recharge. However, it is recommended that there is

always a load present during the drop-out transition to

ensure C

is recharged.

Shutdown and Start-Up (RUN and TK/SS)

The LTC3851A can be shut down using the RUN pin. Pull-

ing this pin below 1.1V disables the controller and most

of the internal circuitry, including the INTV

regulator.

Releasing the RUN pin allows an internal 2µA current to

pull up the pin and enable that control ler. Alternatively,

the RUN pin may be externally pulled up or driven directly

by logic. Be careful not to exceed the absolute maximum

rating of 6V on this pin.

The start-up of the controller’s output voltage, V

OUT

, is

controlled by the voltage on the TK/SS pin. When the

voltage on the TK/SS pin is less than the 0.8V internal

reference, the LTC3851A regulates the V

voltage to the

TK/SS pin voltage instead of the 0.8V reference. This al-

lows the TK/SS pin to be used to program a soft-start by

connecting an external capacitor from the TK/SS pin to

GND. An internal 1µA pull-up current charges this capacitor

creating a voltage ramp on the TK/SS pin. As the TK/SS

voltage rises linearly from 0V to 0.8V (and beyond), the

output voltage V

OUT

rises smoothly from zero to its final

value. Alternatively, the TK/SS pin can be used to cause

the start-up of V

OUT

to track another supply. Typically,

this requires connecting to the TK/SS pin an external

resistor divider from the other supply to ground (see the

Applica tions Information section). When the RUN pin

is pulled low to disable the controller, or when INTV

drops below its undervoltage lockout threshold of 3.2V,

the TK/SS pin is pulled low by an internal MOSFET. When

in undervoltage lockout, the controller is disabled and the

external MOSFETs are held off.

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Continuous Conduction)

The LTC3851A can be enabled to enter high efficiency

Burst Mode operation, constant frequency pulse-skipping

mode or forced continuous conduction mode. To select

forced continuous operation, tie the MODE/PLLIN pin to

INTV

To select pulse-skipping mode of operation, float

the MODE/PLLIN pin or tie it to GND. To select Burst Mode

operation, tie MODE/PLLIN to INTV

through a resistor

no less than 50k, but no greater than 250k.

When the controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-forth of the maximum sense voltage even though

the voltage on the I

pin indicates a lower value. If the

average inductor current is higher than the load current,

LTC3851A

3851afa

operaTion

the error amplifier, EA, will decrease the voltage on the I

pin. When the I

voltage drops below 0.4V, the internal

sleep signal goes high (enabling sleep mode) and both

external MOSFETs are turned off.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s output

begins to rise. When the output voltage drops enough, the

sleep signal goes low, and the controller resumes normal

operation by turning on the top external MOSFET on the

next cycle of the internal oscillator. When a controller is

enabled for Burst Mode operation, the inductor current is

not allowed to reverse. The reverse current comparator,

REV

, turns off the bottom external MOSFET just before the

inductor current reaches zero, preventing it from revers-

ing and going negative. Thus, the controller operates in

discontinuous operation. In forced continuous operation,

the inductor current is allowed to reverse at light loads or

under large transient conditions. The peak inductor cur-

rent is determined by the voltage on the I

pin, just as in

normal operation. In this mode the efficiency at light loads

is lower than in Burst Mode operation. However, continu-

ous mode has the advantages of lower output ripple and

less interference to audio circuitry.

When the MODE/PLLIN pin is connected to GND, the

LTC3851A operates in PWM pulse-skipping mode at light

loads. At very light loads the current comparator, I

CMP

, may

remain tripped for several cycles and force the external top

MOSFET to stay off for the same number of cycles (i.e.,

skipping pulses). The inductor current is not allowed to

reverse (discontinuous operation). This mode, like forced

continuous operation, exhibits low output ripple as well as

low audio noise and reduced RF interference as compared

to Burst Mode operation. It provides higher low current

efficiency than forced continuous mode, but not nearly as

high as Burst Mode operation.

Frequency Selection and Phase-Locked Loop

(FREQ/PLLFLTR and MODE/PLLIN Pins)

The selection of switching frequency is a trade-off between

efficiency and component size. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to main tain low output ripple voltage. The switching fre-

quency of the LTC3851A can be selected using the FREQ/

PLLFLTR pin. If the MODE/PLLIN pin is not being driven

by an external clock source, the FREQ/PLLFLTR pin can

be used to program the controller’s operating frequency

from 250kHz to 750kHz.

A phase-locked loop (PLL) is available on the LTC3851A

to synchronize the internal oscillator to an external clock

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

controller operates in forced continuous mode of operation

when it is synchronized. A series RC should be connected

between the FREQ/PLLFLTR pin and GND to serve as the

PLL’s loop filter. It is suggested that the external clock be

applied before enabling the controller unless a second

resistor is connected in parallel with the series RC network.

The second resistor prevents very low switching frequency

operation if the controller is enabled before the clock.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>10%) as well as other more serious con-

ditions that may overvoltage the output. In such cases,

the top MOSFET is turned off and the bottom MOSFET is

turned on until the overvoltage condition is cleared.

LTC3851A

3851afa

applicaTions inForMaTion

The Typical Application on the first page of this data sheet

is a basic LTC3851A application circuit. The LTC3851A

can be configured to use either DCR (inductor resistance)

sensing or low value resistor sensing. The choice of the

two current sensing schemes is largely a design trade-off

between cost, power consumption and accuracy. DCR

sensing is becoming popular because it saves expensive

current sensing resis tors and is more power efficient,

especially in high current applications. However, current

sensing resistors provide the most accurate current limits

for the controller. Other external component selection

is driven by the load require ment, and begins with the

selection of R

SENSE

(if R

SENSE

is used) and the inductor

value. Next, the power MOSFETs and Schottky diodes are

selected. Finally, input and output capacitors are selected.

The circuit shown on the first page can be configured for

operation up to 38V at V

Current Limit Programming

The I

LIM

pin is a tri-level logic input to set the maximum

current limit of the controller. When I

LIM

is grounded, the

maximum current limit threshold of the current compara-

tor is programmed to be 30mV. When I

LIM

is floated, the

maximum current limit threshold is 50mV. When I

LIM

tied to INTV

, the maximum current limit threshold is

set to 75mV.

SENSE

and SENSE

–

Pins

The SENSE

and SENSE

–

pins are the inputs to the current

comparators. The common mode input voltage range of

the current comparators is 0V to 5.5V. Both SENSE pins

are high impedance inputs with small base currents of

less than 1μA. When the SENSE pins ramp up from 0V

to 1.4V, the small base currents flow out of the SENSE

pins. When the SENSE pins ramp down from 5V to 1.1V,

the small base currents flow into the SENSE pins. The

high impedance inputs to the current comparators allow

accurate DCR sensing. However, care must be taken not

to float these pins during normal operation.

Low Value Resistors Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 1. R

SENSE

is chosen based on the required output

current.

The current comparator has a maximum threshold, V

MAX

determined by the I

LIM

setting. The current comparator

threshold sets the maximum peak of the inductor current,

yielding a maximum average output current, I

MAX

, equal to

the maximum peak value less half the peak-to-peak ripple

current, ∆I

. Allowing a margin of 20% for variations in

the IC and external component values yields:

SENSE

= 0.8 •

MAX

+ ∆I

Inductor DCR Sensing

For applications requiring the highest possible efficiency,

the LTC3851A is capable of sensing the voltage drop

across the inductor DCR, as shown in Figure 2. The

DCR of the inductor represents the small amount of

DC winding resis tance of the copper, which can be less

than 1mΩ for today’s low value, high current inductors.

If the external R1||R2 • C1 time constant is chosen to

be exactly equal to the L/DCR time constant, the voltage

drop across the external capacitor is equal to the voltage

drop across the inductor DCR multiplied by R2/(R1 + R2).

Therefore, R2 may be used to scale the voltage across the

sense terminals when the DCR is greater than the target

sense resistance. Check the manufacturer’s data sheet

for specifications regarding the inductor DCR, in order

to properly dimension the external filter components.

The DCR of the inductor can also be measured using a

good RLC meter.

INTV

BOOST

GND

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE

–

LTC3851A

OUT

SENSE

3851A F01

Figure 1. Using a Resistor to Sense Current with the LTC3851A

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LTC3851AEGN#PBF

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Description:

Switching Voltage Regulators Synchronous Step-Down Switching Regulator Controller

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