LTC1159 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1159

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1159

LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle VOUT/VIN.

To prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

CIN

Required

IRMS

≈

IMAX

[VOUT(VIN

VIN

–

VOUT)]1/2

This formula has a maximum at VIN = 2VOUT, where

IRMS = IMAX/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on only 2000 hours

of life. This makes it advisable to further derate the

capacitor, or to choose a capacitor rated at a higher

temperature than required. Several capacitors may be

paralleled to meet size or height requirements in the

design. An additional 0.1µF ceramic capacitor may also be

required on VIN for high frequency decoupling.

The selection of COUT is driven by the required effective

series resistance (ESR). The ESR of COUT must be less than

twice the value of RSENSE for proper operation of the

LTC1159:

COUT Required ESR < 2RSENSE

Optimum efficiency is obtained by making the ESR equal to

RSENSE. Manufacturers such as Nichicon, Chemicon, and

Sprague should be considered for high performance ca-

pacitors. The OS-CON semiconductor dielectric capacitor

available from Sanyo has the lowest ESR for its size at a

somewhat higher price. Once the ESR requirement for

COUT has been met, the RMS current rating generally far

exceeds the IRIPPLE(P-P) requirement.

In surface mount applications, multiple capacitors may

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

current handling requirements of the application. Alumi-

num 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 TPSD107K010) could be

used. Consult the manufacturer for other specific recom-

mendations.

At low supply voltages, a minimum value of COUT is

suggested to prevent an abnormal low frequency operating

mode (see Figure 4). When COUT is too small, the output

ripple at low frequencies will be large enough to trip the

voltage comparator. This causes the Burst Mode operation

to be activated when the LTC1159 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

L = 50µH

RSENSE = 0.02Ω

600

L = 25µH

RSENSE = 0.02Ω

400

L = 50µH

RSENSE = 0.05Ω

200

0

0

1

2

3

4

5

(VIN – VOUT) VOLTAGE (V)

LTC1159 • TPC04

Figure 4. Minimum Suggested COUT

Load Transient Response

Switching regulators take 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 ITH external components shown in

the Figure 1 circuit will provide adequate compensation for

most applications.

A second, more severe transient is caused by switching in

loads with large (>1µF) supply bypass capacitors. The

10

Share Link: