LTC3851EGN#PBF Datasheet LTC3851EMSE#PBF, LTC3851IMSE#PBF, LTC3851EGN#TRPBF | P10-P12

LTC3851

3851fb

OPERATION

Main Control Loop

The LTC3851 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 I

CMP

resets

the RS latch is controlled by the voltage on the I

pin,

which is the output of the error ampliﬁ er EA. The V

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 V

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 ﬂ oating 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 LTC3851 can be shut down using the RUN pin. Pulling

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 LTC3851 regulates the V

voltage to the

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

allows 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 ﬁ nal

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 LTC3851 can be enabled to enter high efﬁ ciency 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, ﬂ oat 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,

LTC3851

3851fb

OPERATION

the error ampliﬁ er, 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 efﬁ ciency at light

loads is lower than in Burst Mode operation. However,

continuous mode has the advantages of lower output

ripple and less interference to audio circuitry.

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

LTC3851 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

efﬁ ciency 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

efﬁ ciency and component size. Low frequency operation

increases efﬁ ciency by reducing MOSFET switching losses,

but requires larger inductance and/or capacitance to main-

tain low output ripple voltage. The switching frequency

of the LTC3851 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 LTC3851

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 ﬁ lter. 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.

LTC3851

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APPLICATIONS INFORMATION

The Typical Application on the ﬁ rst page of this data sheet

is a basic LTC3851 application circuit. The LTC3851 can

be conﬁ gured 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 efﬁ cient,

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 ﬁ rst page can be conﬁ gured 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 ﬂ oated, 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 1A. When the SENSE pins ramp up from 0V

to 1.4V, the small base currents ﬂ ow out of the SENSE

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

the small base currents ﬂ ow into the SENSE pins. The

high impedance inputs to the current comparators allow

accurate DCR sensing. However, care must be taken not

to ﬂ oat 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

+ΔI

Inductor DCR Sensing

For applications requiring the highest possible efﬁ ciency,

the LTC3851 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

speciﬁ cations regarding the inductor DCR, in order to

properly dimension the external ﬁ lter 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

–

LTC3851

OUT

SENSE

3851 F01

Figure 1. Using a Resistor to Sense Current with the LTC3851

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