LTC3810

100V Current Mode

Synchronous Switching

Regulator Controller

line and load transient response. Large 1Ω gate drivers

allow the LTC3810 to drive multiple MOSFETs for higher

current applications. The operating frequency is selected

by an external resistor and is compensated for variations

in VIN and can also be synchronized to an external clock

current to 240μA.

n High Voltage Operation: Up to 100V

n Large 1Ω Gate Drivers

n No Current Sense Resistor Required

n Dual N-Channel MOSFET Synchronous Drive

n Extremely Fast Transient Response

n ±0.5% 0.8V Voltage Reference

n Programmable Output Voltage Tracking/Soft-Start

n Generates 10V Driver Supply from Input Supply

n Synchronizable to External Clock

n Selectable Pulse Skip Mode Operation

High Effi ciency High Voltage Step-Down Converter Effi ciency vs Load Current

LTC3810

BOOST ................................................ –0.3V to 114V

BGRTN ....................................................... –5V to 0V

EXTVCC .................................................. –0.3V to 15V

(NDRV - INTVCC) Voltage ........................... –0.3V to 10V

SW, SENSE+ Voltage ................................... –1V to 100V

ION Voltage ............................................... –0.3V to 100V

SS/TRACK Voltage ....................................... –0.3V to 5V

PGOOD Voltage ............................................ –0.3V to 7V

UVIN Voltages ........................................ –0.3V to 14V

(Note 1)

The

temperature range, otherwise specifi cations are at TA = 25°C (Note 2), INTVCC = DRVCC = VBOOST = VON = VRNG = SHDN = UVIN =

VEXTVCC = VNDRV = 10V, VMODE/SYNC = VSENSE+ = VSENSE– = VBGRTN = VSW =0V, unless otherwise specifi ed.

LTC3810

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Load Transient Response Start-Up

Fault Timeout Operation

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On-Time vs ION Current

Feedback Reference Voltage

vs Temperature

3810 G18

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INTVCC Current vs Temperature

INTVCC Shutdown Current

vs Temperature

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ION (Pin 1): On-Time Current Input. Tie a resistor from VIN

set the switching frequency.

the on-time comparator. Tying this pin to the output voltage

or an external resistive divider from the output makes the

on-time proportional to VOUT. The comparator defaults to

0.7V when the pin is grounded and defaults to 2.4V when

the power good output is pulled to ground.

to regulate a second output voltage.

(zero current).

VFB (Pin 9): Feedback Input. Connect VFB through a resistor

divider network to VOUT to set the output voltage.

nents should connect to this ground and eventually connect

to PGND at one point.

rent to 240μA.

gate drive are drawn from EXTVCC.

INTVCC (Pin 17): Main Supply Pin. All internal circuits

INTVCC should be bypassed to ground (Pin 10) with at

DRVCC (Pin 18): Driver Supply Pin. DRVCC supplies power

LTC3810

transition losses. MOSFET input capacitance is a combi-

(Figure 6).

specifi ed VDS values. A way to estimate the CMILLER term

MOSFETs at maximum output current are given by:

VIN

IMAX

VIN

where ρT is the temperature dependency of RDS(ON), RDR

the technique described above.

MOSFET losses are greatest at high input voltage when

LTC3810

the period. Since there is no transition loss term in the

synchronous MOSFET, optimal effi ciency is obtained by

Operating Frequency

is set by the current out of the ION pin and the voltage at

on-time inversely proportional to VIN. For a step-down

LTC3810

of the voltage change at the ITH pin to the VON pin as

Inductor Selection

Given the desired input and output voltages, the induc-

VIN

Lower ripple current reduces core losses in the inductor,

ripple. Highest effi ciency operation is obtained at low

does not exceed a specifi ed maximum, the inductance

should be chosen according to:

Schottky Diode D1 Selection

LTC3810

that these components be placed adjacently. The diode can

Input Capacitor Selection

the maximum RMS current is given by:

IO(MAX)/2. This simple worst-case condition is commonly

used for design because even signifi cant deviations do not

offer much relief. Note that the ripple current ratings from

in voltages above 30V, ceramics or aluminum electrolytics

must be used for regulators with input supplies above 30V.

Ceramic capacitors have the advantage of very low ESR

and can handle high RMS current, but ceramics with high

voltage ratings (> 50V) are not available with more than

a few microfarads of capacitance. Furthermore, ceram-

and RMS current. If the RMS current cannot be handled

the percentage of RMS current that will be supplied by the

aluminum capacitor is reduced to approximately:

where RESR is the ESR of the aluminum capacitor and C

is the overall capacitance of the ceramic capacitors. Using

an aluminum electrolytic with a ceramic also helps damp

Since ∆IL increases with input voltage, the output ripple

and Sanyo should be considered for high performance

throughhole capacitors. The OS-CON (organic semicon-

ductor dielectric) capacitor available from Sanyo has the

lowest product of ESR and size of any aluminum electroly-

tic at a somewhat higher price. An additional ceramic

capacitor in parallel with OS-CON capacitors is recom-

mended to reduce the effect of their lead inductance.

In surface mount applications, multiple capacitors placed

LTC3810

that have been surge tested for use in switching power

supplies. Several excellent surge-tested choices are the

SP and Sanyo POSCAPs.

LTC3810

ing formulas:

270μA

and VTH is the threshold voltage of the MOSFET.

an NMOS to reduce VT to ~1.4V.

PMOSFET(MAX), in the MOSFET is exceeded and leave the

times are a function of the TRACK/SS capacitor value.

LTC3810

The external NMOS for the linear regulator should be a

standard 3V threshold type (i.e., not a logic level threshold).

charging current for this capacitor is approximately:

should be chosen so that capacitor charging does not

damage the NMOS. Excessive values of capacitor are

unnecessary and should be avoided. Typically values in

the 1μF to 10μF work well.

One more design requirement for this mode is the minimum

soft-start capacitor value. The fault timeout is enabled

C

Mode 2 should be used if VOUT is outside of the 6.7V <

EXTVCC < 15V operating range and the extra complexity

grounded and the NMOS is chosen to handle the worst-

case power dissipation:

disabled by choosing

R

value for VIN(MIN) (see Equation 1), fault timeout operation

Using Trickle Charge Mode

Trickle charge mode is selected by shorting NDRV and

INTVCC and connecting EXTVCC to VOUT. Trickle charge

mode has the advantage of not requiring an external

MOSFET but takes longer to start up due to slow charge

up of CINTVCC and CDRVCC through RPULLUP (tDELAY = 0.77

• RPULLUP • CDRVCC) and usually requires larger INTVCC/

up, CINTVCC/CDRVCC must be chosen large enough so that

6.7V before CINTVCC/CDRVCC discharges below the falling

IMAX

VOUT(REG)

where IG is the gate drive current = (f)(QG(TOP) + QG(BOTTOM))

Max RPULLUP < (VIN(MIN)–12V)/IQ,SHUTDOWN

LTC3810

Using an External Supply Connected to the INTVCC/

normal operation and from the external NMOS from VIN

during start-up or short-circuit. Using the EXTVCC pin in

to what would be possible when deriving this power

continuously from the typically much higher VIN voltage.

The EXTVCC connection also allows the power supply to

be confi gured in trickle charge mode in which it starts up

to INTVCC to charge up the INTVCC capacitor. As soon as

to be powered continuously from an external NMOS

10% at high input voltages.

The junction temperature can be estimated from the

equations given in Note 2 of the Electrical Characteristics

as follows:

T

LTC3810

FEEDBACK LOOP/COMPENSATION

these components affect loop behavior and must be ac-

controlled current source from ITH to VOUT and has a gain

equal to (IMAXROUT)/1.2V. It has fairly benign AC behavior

boost to achieve the desired phase margin and the only

requirement of the compensation will be to guarantee that

LTC3810

Feedback Component Selection

Selecting the R and C values for a typical Type 2 or Type 3

loop is a nontrivial task. The applications shown in this

a working system. To measure the modulator gain and

measurements or damage the analyzer.

ITH with gain in dB and

transfer function of the modulator:

1.2 •RDS(ON)

LTC3810

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

or ringing that would indicate a stability problem.

12V ±5%, IOUT(MAX) = 10A, f = 250kHz. First, calculate the

timing resistor with VON = INTVCC:

C

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