MAX1652EEE datasheet(19/28 Pages) MAXIM

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High-Efficiency, PWM, Step-Down

DC-DC Controllers in 16-Pin QSOP

______________________________________________________________________________________

output voltage as determined by the turns ratio is +15V,

the feedback resistor ratio should be set to produce

about +13.5V; otherwise, the SECFB one-shot might be

triggered unintentionally, causing an unnecessary

increase in supply current and output noise. In negative-

output (MAX1654) applications, the resistor-divider acts

as a load on the internal reference, which in turn can

cause errors at the main output. Avoid overloading REF

(see the Reference Load-Regulation Error vs. Load

Current graph in the

Typical Operating Characteristics).

100k

Ω is a good value for R3 in MAX1654 circuits.

Output current on secondary winding applications is

limited at low input voltages. See the MAX1652

Maximum Secondary Output Current vs. Supply Voltage

graph in the Typical Operating Characteristics for data

from the application circuit of Figure 8.

Soft-Start Circuit (SS)

Soft-start allows a gradual increase of the internal cur-

rent-limit level at start-up for the purpose of reducing

input surge currents, and perhaps for power-supply

sequencing. In shutdown mode, the soft-start circuit

holds the SS capacitor discharged to ground. When

SHDN goes high, a 4µA current source charges the SS

capacitor up to 3.2V. The resulting linear ramp wave-

form causes the internal current-limit level to increase

proportionally from 0 to 100mV. The main output capaci-

tor thus charges up relatively slowly, depending on the

SS capacitor value. The exact time of the output rise

depends on output capacitance and load current and is

typically 1ms per nanofarad of soft-start capacitance.

With no SS capacitor connected, maximum current limit

is reached within 10µs.

Shutdown

Shutdown mode (SHDN = 0V) reduces the V+ supply

current to typically 3µA. In this mode, the reference and

VL are inactive. SHDN is a logic-level input, but it can

be safely driven to the full V+ range. Connect SHDN to

V+ for automatic start-up. Do not allow slow transitions

(slower than 0.02V/µs) on SHDN.

MAX1652

GND

REF

SYNC

SECFB

Si9410

EC11FS1

T1 = TRANSPOWER TTI5870

* = OPTIONAL, MAY NOT BE NEEDED

CMPSH

-3A

1N5819

VIN (6.5V TO 18V)

+15V AT

250mA

+5V

AT 3A

ON/OFF

CSL

CSH

BST

V+

PGND

SHDN

0.33

µF

4.7

µF

2.5V

220

µF

10V

220

µF

10V

0.1

µF

µF, 35V

0.01

µF

20m

Ω

Ω*

4700pF*

µH

2.2:1

49.9k, 1%

210k, 1%

0.01

µF

(OPTIONAL)

18V

1/4 W

4.7

µF

Figure 8. 5V/15V Dual-Output Application Circuit (MAX1652)

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