LTC3883 Datasheet, PDF(45/112 Page) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

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LTC3883 Datasheet, PDF (45/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

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LTC3883/LTC3883-1

Applications Information

voltage exceeds the RDS(ON) test voltage for the MOSFETs

which is typically 4.5V for logic level devices. The UVLO

on INTVCC (EXTVCC) is set to approximately 4V. Both the

LTC3883 and LTC3883-1 are valid for this configuration.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pin supplies the gate drive voltages for the topside MOS-

FETs. Capacitor CB in the Block Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on,

the driver places the CB voltage across the gate source

of the desired MOSFET. This enhances the MOSFET and

turns on the topside switch. The switch node voltage, SW,

rises to VIN and the BOOST pin follows. With the topside

MOSFET on, the boost voltage is above the input supply:

VBOOST = VIN + VINTVCC. The value of the boost capacitor

CB needs to be 100 times that of the total input capacitance

of the topside MOSFET(s). The reverse breakdown of the

external Schottky diode must be greater than VIN(MAX).

When adjusting the gate drive level, the final arbiter is the

total input current for the regulator. If a change is made

and the input current decreases, then the efficiency has

improved. If there is no change in input current, then there

is no change in efficiency.

PWM jitter has been observed in some designs operating

at higher VIN/VOUT ratios. This jitter does not substantially

affect the circuit accuracy. Referring to Figure 24, PWM

jitter can be removed by inserting a series resistor with a

value of 1Î© to 5Î© between the cathode of the diode and

the BOOST pin. A resistor case size of 0603 or larger is

recommended to reduce ESL and achieve the best results.

VIN

BOOST

TGATE

LTC3883/

LTC3883-1

INTVCC/EXTVCC

BGATE

1Î© TO 5Î©

0.2ÂµF

CINTVCC

10ÂµF

PGND

VIN

3883 F24

Figure 24. Boost Circuit to Minimize PWM Jitter

Undervoltage Lockout

The LTC3883 is initialized by an internal threshold-based

UVLO where VIN must be approximately 4V and INTVCC/

EXTVCC, VDD33, VDD25 must be within approximately 20%

of the regulated values. In addition, VDD33 must be within

approximately 7% of the targeted value before the RUN

pin is released. After the part has initialized, an additional

comparator monitors VIN. The VIN_ON threshold must

be exceeded before the power sequencing can begin.

When VIN drops below the VIN_OFF threshold, the RUN

pin will be pulled low and VIN must increase above the

VIN_ON threshold before the controller will restart. The

normal start-up sequence will be allowed after the VIN_ON

threshold is crossed.

It is possible to program the contents of the NVM in the

application if the VDD33 supply is externally driven. This will

activate the digital portion of the LTC3883 without engaging

the high voltage sections. PMBus communications are valid

in this supply configuration. If VIN has not been applied to

the LTC3883, bit 3 (NVM Not Initialized)in MFR_COMMON

will be asserted low. If this condition is detected, the part

will only respond to addresses 5A and 5B. To initialize

the part issue the following set of commands: global

address 0x5B command 0xBD data 0x2B followed by

global address 5B command 0xBD and data 0xC4. The

part will now respond to the correct address. Configure

the part as desired then issue a STORE_USER_ALL. When

VIN is applied a MFR_RESET command must be issued to

allow the PWM to be enabled and valid ADC conversions

to be read.

CIN and COUT Selection

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

maximum RMS current of one channel must be used. The

maximum RMS capacitor current is given by:

( )( ) CIN

Required

IRMS

IMAX

VIN

ï£®ï£°

VOUT

VIN â VOUT ï£¹ï£»1/2

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant deviations

3883f

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