LTC1625IGN datasheet(16/24 Pages) LINER

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LTC1625

where L1, L2, etc. are the individual losses as a percentage

of input power. It is often useful to analyze individual

losses to determine what is limiting the efficiency and

which change would produce the most improvement.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1625 circuits:

1. INTVCC current. This is the sum of the MOSFET driver

and control currents. The driver current results from

switching the gate capacitance of the power MOSFETs.

Each time a MOSFET gate is switched on and then off,

a packet of gate charge Qg moves from INTVCC to

ground. The resulting current out of INTVCC is typically

much larger than the control circuit current. In continu-

ous mode, IGATECHG = f(Qg(TOP) + Qg(BOT)).

By powering EXTVCC from an output-derived source,

the additional VIN current resulting from the driver and

control currents will be scaled by a factor of Duty Cycle/

Efficiency. For example, in a 20V to 5V application at

400mA load, 10mA of INTVCC current results in ap-

proximately 3mA of VIN current. This reduces the loss

from 10% (if the driver was powered directly from VIN)

to about 3%.

2. DC I2R Losses. Since there is no separate sense resis-

tor, DC I2R losses arise only from the resistances of the

MOSFETs and inductor. In continuous mode the aver-

age output current flows through L, but is “chopped”

between the top MOSFET and the bottom MOSFET. If

the two MOSFETs have approximately the same RDS(ON),

then the resistance of one MOSFET can simply be

summed with the resistance of L to obtain the DC I2R

loss. For example, if each RDS(ON) = 0.05Ω and RL =

0.15

Ω, then the total resistance is 0.2Ω. This results in

losses ranging from 2% to 8% as the output current

increases from 0.5A to 2A for a 5V output. I2R losses

cause the efficiency to drop at high output currents.

3. Transition losses apply only to the topside MOSFET,

and only when operating at high input voltages (typi-

cally 20V or greater). Transition losses can be esti-

mated from:

Transition Loss = (1.7)(VIN

2)(IO(MAX))(CRSS)(f)

APPLICATIONS INFORMATION

4. LTC1625 VIN supply current. The VIN current is the DC

supply current to the controller excluding MOSFET gate

drive current. Total supply current is typically about

850

µA. If EXTVCC is connected to 5V, the LTC1625 will

draw only 330

µAfromVINandtheremaining520µAwill

come from EXTVCC. VIN current results in a small

(< 1%) loss which increases with VIN.

Other losses including CIN and COUT ESR dissipative

losses, Schottky conduction losses during dead time

and inductor core losses, generally account for less

than 2% total additional loss.

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in DC (resistive) load

current. When a load step occurs, VOUT immediately shifts

by an amount equal to (

∆ILOAD)(ESR), where ESR is the

effective series resistance of COUT, and COUT begins to

charge or discharge. The regulator loop acts on the

resulting feedback error signal to return VOUT to its steady-

state value. During this recovery time VOUT can be moni-

tored for overshoot or ringing which would indicate a

stability problem. The ITH pin external components shown

in Figure 1 will provide adequate compensation for most

applications.

A second, more severe transient is caused by connecting

loads with large (> 1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive in order

to limit the inrush current to the load.

Automotive Considerations: Plugging into the

Cigarette Lighter

As battery-powered devices go mobile, there is a natural

interest in plugging into the cigarette lighter in order to

conserve or even recharge battery packs during opera-

tion. But before you connect, be advised: you are plug-

ging into the supply from hell. The main battery line in an

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