LTC1148CN 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1148CN

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

UU W U

APPLICATIO S I FOR ATIO

In surface mount applications multiple capacitors may

have to be paralleled to meet the capacitance, ESR, or

RMS current handling requirements of the application.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount configurations. In the

case of tantalum, it is critical that the capacitors are surge

tested for use in switching power supplies. An excellent

choice is the AVX TPS series of surface mount tantalums,

available in case heights ranging from 2mm to 4mm. For

example, if 200µF/10V is called for in an application

requiring 3mm height, two AVX 100µF/10V (P/N TPSD

107K010) could be used. Consult the manufacturer for

other specific recommendations.

At low supply voltages, a minimum capacitance at COUT

is needed to prevent an abnormal low frequency oper-

ating mode (see Figure 4). When COUT is made too

small, the output ripple at low frequencies will be large

enough to trip the voltage comparator. This causes

Burst Mode operation to be activated when the LTC1148

series would normally be in continuous operation. The

effect is most pronounced with low values of RSENSE

and can be improved by operating at higher frequencies

with lower values of L. The output remains in regulation

at all times.

1000

800

600

400

L = 50µH

RSENSE = 0.02Ω

L = 25µH

RSENSE = 0.02Ω

200

0

0

L = 50µH

RSENSE = 0.05Ω

1

2

3

4

5

(VIN – VOUT) VOLTAGE (V)

LTC1148 • F04

Figure 4. Minimum Value of COUT

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

LTC1148

LTC1148-3.3/LTC1148-5

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

current. When a load step occurs, VOUT shifts by an

amount equal to ∆ILOAD • ESR, where ESR is the effective

series resistance of COUT. ∆ILOAD also begins to charge

or discharge COUT until the regulator loop adapts to the

current change and returns VOUT to its steady state

value. During this recovery time VOUT can be monitored

for overshoot or ringing which would indicate a stability

problem. The Pin 6 external components shown in the

Figure 1 circuit will prove adequate compensation for

most applications.

A second, more severe transient is caused by switching in

loads with large (>1µF) 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 so that

the load rise time is limited to approximately 25 • CLOAD.

Thus a 10µF capacitor would require a 250µs rise time,

limiting the charging current to about 200mA.

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. (For high efficiency circuits only small

errors are incurred by expressing losses as a percentage

of output power).

Although all dissipative elements in the circuit produce

losses, three main sources usually account for most of the

losses in LTC1148 series circuits: 1) LTC1148 DC bias

current, 2) MOSFET gate charge current, and 3) I2R

losses.

1. The DC supply current is the current which flows into

VIN Pin 3 less the gate charge current. For VIN = 10V the

11

Share Link: