LT3503 查看數據表（PDF） - Linear Technology

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LT3503

1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3503

APPLICATIO S I FOR ATIO

damps the circuit and eliminates the voltage overshoot.

The extra capacitor improves low frequency ripple filtering

and can slightly improve the efficiency of the circuit,

though it is likely to be the largest component in the circuit.

An alternative solution is shown in Figure 10c. A 1Ω

resistor is added in series with the input to eliminate the

voltage overshoot (it also reduces the peak input current).

A 0.1µF capacitor improves high frequency filtering. This

solution is smaller and less expensive than the electrolytic

capacitor. For high input voltages its impact on efficiency

is minor, reducing efficiency less than one half percent for

a 5V output at full load operating from 20V.

Frequency Compensation

The LT3503 uses current mode control to regulate the

output. This simplifies loop compensation. In particular,

the LT3503 does not require the ESR of the output

capacitor for stability allowing the use of ceramic capaci-

tors to achieve low output ripple and small circuit size.

Figure 11 shows an equivalent circuit for the LT3503

control loop. The error amp is a transconductance ampli-

fier with finite output impedance. The power section,

consisting of the modulator, power switch and inductor,

is modeled as a transconductance amplifier generating an

output current proportional to the voltage at the VC node.

Note that the output capacitor integrates this current, and

that the capacitor on the VC node (CC) integrates the error

amplifier output current, resulting in two poles in the loop.

RC provides a zero. With the recommended output

capacitor, the loop crossover occurs above the RCCC zero.

This simple model works well as long as the value of the

inductor is not too high and the loop crossover frequency

is much lower than the switching frequency. With a larger

ceramic capacitor (very low ESR), crossover may be

lower and a phase lead capacitor (CPL) across the feed-

back divider may improve the phase margin and transient

response. Large electrolytic capacitors may have an ESR

large enough to create an additional zero, and the phase

lead may not be necessary.

If the output capacitor is different than the recommended

capacitor, stability should be checked across all operating

conditions, including load current, input voltage and tem-

perature. The LT1375 data sheet contains a more thor-

ough discussion of loop compensation and describes how

to test the stability using a transient load.

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 12 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents flow in the LT3503’s VIN and SW pins, the catch

diode (D1) and the input capacitor (C2). The loop formed

by these components should be as small as possible and

tied to system ground in only one place. These compo-

nents, along with the inductor and output capacitor,

should be placed on the same side of the circuit board, and

their connections should be made on that layer. Place a

LT3503

0.8V

CURRENT MODE

– POWER STAGE

gm =

SW

+1.1A/V

R1

VC

gm =

200µA/V

RC

24k

CC

100pF

ERROR

AMPLIFIER

2M

FB

780mV

GND

R2

OUT

CPL

ESR

+

C1

C1

16

3503 F11

Figure 11. Model for Loop Response

3503f

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