LTC3890HUH-2#TRPBF Datasheet LTC3890IUH-2#PBF, LTC3890HUH-2#PBF, LTC3890MPUH-2#TRPBF | P13-P15

LTC3890-2

38902f

OPERATION

(Refer to the Functional Diagram)

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Forced Continuous Mode)

(PLLIN/MODE Pin)

The LTC3890-2 can be enabled to enter high efficiency

Burst Mode operation, constant frequency pulse-skipping

mode, or forced continuous conduction mode at low load

currents. To select Burst Mode operation, tie the PLLIN/

MODE pin to a DC voltage below 0.8V (e.g., SGND). To

select forced continuous operation, tie the PLLIN/MODE

pin to INTV

. To select pulse-skipping mode, tie the

PLLIN/MODE pin to a DC voltage greater than 1.2V and

less than INTV

– 1.3V.

When a controller is enabled for Burst Mode operation, the

minimum peak current in the inductor is set to approxi-

mately 25% of the maximum sense voltage even though

the voltage on the ITH pin indicates a lower value. If the

average inductor current is higher than the load current,

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

ITH pin. When the ITH voltage drops below 0.425V, the

internal sleep signal goes high (enabling sleep mode)

and both external MOSFETs are turned off. The ITH pin is

then disconnected from the output of the EA and parked

at 0.450V.

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current that the LTC3890-2 draws.

If one channel is shut down and the other channel is in

sleep mode, the LTC3890-2 draws only 50µA of quiescent

current. If both channels are in sleep mode, the LTC3890-

2 draws only 60µA of quiescent current. 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 ITH pin

is reconnected to the output of the EA, 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 cur-

rent comparator, IR, turns off the bottom external MOSFET

just before the inductor current reaches zero, preventing

it from reversing and going negative. Thus, the controller

operates in discontinuous operation.

In forced continuous operation or clocked by an external

clock source to use the phase-locked loop (see Frequency

Selection and Phase-Locked Loop section), the induc-

tor current is allowed to reverse at light loads or under

large transient conditions. The peak inductor current is

determined by the voltage on the ITH pin, just as in normal

operation. In this mode, the efficiency at light loads is lower

than in Burst Mode operation. However, continuous opera-

tion has the advantage of lower output voltage ripple and

less interference to audio circuitry. In forced continuous

mode, the output ripple is independent of load current.

When the PLLIN/MODE pin is connected for pulse-skipping

mode, the LTC3890-2 operates in PWM pulse-skipping

mode at light loads. In this mode, constant frequency

operation is maintained down to approximately 1% of

designed maximum output current. At very light loads, the

current comparator, ICMP, 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 and PLLIN/MODE 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 maintain low output ripple voltage.

The switching frequency of the LTC3890-2’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 SGND, tied to

INTV

or programmed through an external resistor. Tying

FREQ to SGND selects 350kHz while tying FREQ to INTV

selects 535kHz. Placing a resistor between FREQ and SGND

allows the frequency to be programmed between 50kHz

and 900kHz, as shown in Figure 10.

LTC3890-2

38902f

OPERATION

(Refer to the Functional Diagram)

A phase-locked loop (PLL) is available on the LTC3890-2

to synchronize the internal oscillator to an external clock

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

LTC3890-2’s phase detector adjusts the voltage (through

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

turn-on of controller 1’s external top MOSFET to the ris-

ing edge of the synchronizing signal. Thus, the turn-on

of controller 2’s external top MOSFET is 180 degrees out

of phase to the rising edge of the external clock source.

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. 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 phase-locked loop is from

approximately 55kHz to 1MHz, with a guarantee to be

between 75kHz and 850kHz. In other words, the LTC3890-

2’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).

When synchronized to an external clock using the PLLIN/

MODE pin, the LTC3890-2 operates in pulse-skipping

mode at light loads.

PolyPhase Applications (CLKOUT and PHASMD Pins)

The LTC3890-2 features two pins (CLKOUT and PHASMD)

that allow other controller ICs to be daisy-chained with the

LTC3890-2 in PolyPhase applications. The clock output

signal on the CLKOUT pin can be used to synchronize

additional power stages in a multiphase power supply

solution feeding a single, high current output or multiple

separate outputs. The PHASMD pin is used to adjust the

phase of the CLKOUT signal as well as the relative phases

between the two internal controllers, as summarized in

Table 1. The phases are calculated relative to the zero

degrees phase being defined as the rising edge of the top

gate driver output of controller 1 (TG1).

Table 1

PHASMD

CONTROLLER 2 PHASE CLKOUT PHASE

GND 180° 60°

Floating 180° 90°

INTV

240° 120°

Power Good (PGOOD1 and PGOOD2) Pins

Each PGOOD pin is connected to an open drain of an internal

N-channel MOSFET. The MOSFET turns on and pulls the

PGOOD pin low when the corresponding V

pin voltage is

not within ±10% of the 0.8V reference voltage. The PGOOD

pin is also pulled low when the corresponding RUN pin

is low (shut down). When the V

pin voltage is within

the ±10% requirement, the MOSFET is turned off and the

pin is allowed to be pulled up by an external resistor to a

source no greater than 6V.

Theory and Benefits of 2-Phase Operation

Why the need for 2-phase operation? Up until the 2-phase

family, constant-frequency dual switching regulators

operated both channels in phase (i.e., single phase

operation). This means that both switches turned on at

the same time, causing current pulses of up to twice the

amplitude of those for one regulator to be drawn from the

input capacitor and battery. These large amplitude current

pulses increased the total RMS current flowing from the

input capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and battery.

LTC3890-2

38902f

OPERATION

(Refer to the Functional Diagram)

With 2-phase operation, the two channels of the dual

switching regulator are operated 180 degrees out-of-phase.

This effectively interleaves the current pulses drawn by the

switches, greatly reducing the overlap time where they add

together. The result is a significant reduction in total RMS

input current, which in turn allows less expensive input

capacitors to be used, reduces shielding requirements for

EMI and improves real world operating efficiency.

Figure 1 compares the input waveforms for a representative

single-phase dual switching regulator to the LTC3890-2

2-phase dual switching regulator. An actual measurement of

the RMS input current under these conditions shows that

2-phase operation dropped the input current from 2.53A

RMS

to 1.55A

RMS

. While this is an impressive reduction in itself,

remember that the power losses are proportional to I

RMS

meaning that the actual power wasted is reduced by a fac-

tor of 2.66. The reduced input ripple voltage also means

less power is lost in the input power path, which could

include batteries, switches, trace/connector resistances

and protection circuitry. Improvements in both conducted

and radiated EMI also directly accrue as a result of the

reduced RMS input current and voltage.

Of course, the improvement afforded by 2-phase opera-

tion is a function of the dual switching regulator’s relative

duty cycles which, in turn, are dependent upon the input

voltage V

(Duty Cycle = V

OUT

). Figure 2 shows how

the RMS input current varies for single-phase and 2-phase

operation for 3.3V and 5V regulators over a wide input

voltage range.

It can readily be seen that the advantages of 2-phase op-

eration are not just limited to a narrow operating range,

for most applications is that 2-phase operation will reduce

the input capacitor requirement to that for just one chan-

nel operating at maximum current and 50% duty cycle.

Figure 2. RMS Input Current Comparison

INPUT VOLTAGE (V)

INPUT RMS CURRENT (A)

3.0

2.5

2.0

1.5

1.0

0.5

10 20 30 40

38902 F02

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

= 5V/3A

= 3.3V/3A

IN(MEAS)

= 2.53A

RMS

IN(MEAS)

= 1.55A

RMS

38902 F01

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators

Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows

Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency

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LTC3890HUH-2#TRPBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators 60V Low IQ, Dual Output Synchronous Step-Down Controller

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LTC3890HUH-2#TRPBF

LTC3890HUH-2#TRPBF

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