LTM4607 datasheet(13/30 Pages) LINER

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LTC3780

3780ff

For more information www.linear.com/LTC3780

operaTion

MAIN CONTROL LOOP

The LTC3780 is a current mode controller that provides an

output voltage above, equal to or below the input voltage.

The LTC proprietary topology and control architecture em-

ploys a current-sensing resistor in buck or boost modes.

The sensed inductor current is controlled by the voltage

on the ITH pin, which is the output of the amplifier EA. The

VOSENSE pin receives the voltage feedback signal, which

is compared to the internal reference voltage by the EA.

The top MOSFET drivers are biased from floating boost-

strap capacitors CAandCB(Figure11),whicharenormally

rechargedthroughanexternaldiodewhenthetopMOSFET

is turned off. Schottky diodes across the synchronous

switch D and synchronous switch B are not required, but

provide a lower drop during the dead time. The addition of

the Schottky diodes will typically improve peak efficiency

by 1% to 2% at 400kHz.

The main control loop is shut down by pulling the RUN

pin low. When the RUN pin voltage is higher than 1.5V, an

internal 1.2µA current source charges soft-start capacitor

CSS at the SS pin. The ITH voltage is then clamped to the

SS voltage while CSS is slowly charged during start-up.

This “soft-start” clamping prevents abrupt current from

being drawn from the input power supply.

POWER SWITCH CONTROL

Figure 1 shows a simplified diagram of how the four

power switches are connected to the inductor, VIN, VOUT

and GND. Figure 2 shows the regions of operation for the

LTC3780 as a function of duty cycle D. The power switches

are properly controlled so the transfer between modes is

continuous. When VIN approaches VOUT, the buck-boost

region is reached; the mode-to-mode transition time is

typically 200ns.

Buck Region (VIN > VOUT)

Switch D is always on and switch C is always off during

this mode. At the start of every cycle, synchronous switch

B is turned on first. Inductor current is sensed when

synchronous switch B is turned on. After the sensed in-

ductor current falls below the reference voltage, which is

proportional to VITH, synchronous switch B is turned off

and switch A is turned on for the remainder of the cycle.

switches A and B will alternate, behaving like a typical

synchronous buck regulator. The duty cycle of switch A

increases until the maximum duty cycle of the converter

in buck mode reaches DMAX_BUCK, given by:

DMAX_BUCK = 100% – DBUCK-BOOST

where DBUCK-BOOST = duty cycle of the buck-boost switch

range:

DBUCK-BOOST = (200ns • f) • 100%

and f is the operating frequency in Hz.

Figure 3 shows typical buck mode waveforms. If VIN

approaches VOUT, the buck-boost region is reached.

Buck-Boost (VIN @ VOUT)

When VIN is close to VOUT, the controller is in buck-boost

mode. Figure 4 shows typical waveforms in this mode.

Every cycle, if the controller starts with switches B and D

turned on, switches A and C are then turned on. Finally,

switches A and D are turned on for the remainder of the

time. If the controller starts with switches A and C turned

TG2

BG2

TG1

BG1

RSENSE

3780 F01

SW2

SW1

VIN

VOUT

A ON, B OFF

PWM C, D SWITCHES

D ON, C OFF

PWM A, B SWITCHES

FOUR SWITCH PWM

98%

DMAX

BOOST

DMIN

BUCK

DMIN

BOOST

DMAX

BUCK

BOOST REGION

BUCK REGION

BUCK/BOOST REGION

3780 F02

Figure 1. Simplified Diagram of the Output Switches

Figure 2. Operating Mode vs Duty Cycle

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