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LTC3624/LTC3624-2

36242fc

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applicaTions inForMaTion

shown in Figure 1. Capacitor CFF provides phase lead by

creating a high frequency zero with R2, which improves

the phase margin.

Theoutputvoltagesettlingbehaviorisrelatedtothestability

of the closed-loop system and will demonstrate the actual

overall supply performance. For a detailed explanation of

optimizing the compensation components, including a

review of control loop theory, refer to application Note 76.

Insomeapplications,amoreseveretransientcanbecaused

by switching in loads with large (>1µF) input capacitors.

The discharge input capacitors are effectively put in paral-

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

can deliver enough current to prevent this problem if the

switchconnectingtheloadhaslowresistanceandisdriven

quickly. The solution is to limit the turn-on speed of the

load switch driver. A Hot Swap™ controller is designed

specifically for this purpose and usually incorporates

current limiting, short-circuit protection and soft-starting.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can

be expressed as:

% Efficiency = 100% – (L1 + L2 + L3 +…)

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

age of input power. Although all dissipative elements in

the circuit produce losses, three main sources usually

account for most of the losses in LTC3624/LTC3624-2

circuits: 1) I2R losses, 2) switching and biasing losses,

3) other losses.

1. I2R losses are calculated from the DC resistances of

the internal switches, RSW, and external inductor, RL.

In continuous mode, the average output current flows

through inductor L but is “chopped” between the

internal top and bottom power MOSFETs. Thus, the

series resistance looking into the SW pin is a function

of both top and bottom MOSFET RDS(ON) and the duty

cycle (DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

TheRDS(ON)forboththetopandbottomMOSFETscanbe

obtained from the Typical Performance Characteristics

curves. Thus to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

2. The switching current is the sum of the MOSFET driver

and control currents. The power MOSFET driver current

resultsfromswitchingthegatecapacitanceofthepower

MOSFETs. Each time a power MOSFET gate is switched

from low to high to low again, a packet of charge dQ

moves from IN to ground. The resulting dQ/dt is a cur-

rent out of IN that is typically much larger than the DC

control bias current. In continuous mode, IGATECHG =

f(QT + QB), where QT and QB are the gate charges of

the internal top and bottom power MOSFETs and f is

the switching frequency. The power loss is thus:

Switching Loss = IGATECHG • VIN

The gate charge loss is proportional to VIN and f and

thus their effects will be more pronounced at higher

supply voltages and higher frequencies.

3. Other “hidden” losses such as transition loss and cop-

per trace and internal load resistances can account for

additional efficiency degradations in the overall power

system. It is very important to include these “system”

level losses in the design of a system. Transition loss

arises from the brief amount of time the top power

MOSFET spends in the saturated region during switch

node transitions. The LTC3624/LTC3624-2 internal

power devices switch quickly enough that these losses

are not significant compared to other sources. These

losses plus other losses, including diode conduction

losses during dead-time and inductor core losses,

generally account for less than 2% total additional loss.

Thermal Conditions

Inamajorityofapplications,theLTC3624/LTC3624-2does

not dissipate much heat due to its high efficiency and

low thermal resistance of its exposed pad DFN package.

However, in applications where the LTC3624/LTC3624-2

is running at high ambient temperature, high VIN, high

switching frequency, and maximum output current load,

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