MAX797CSLASHD datasheet(18/32 Pages) MAXIM

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Step-Down Controllers with

Synchronous Rectifier for CPU Power

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MAX796), a 1µs one-shot is triggered that extends the

low-side switch’s on-time beyond the point where the

inductor current crosses zero (in discontinuous mode).

This causes the inductor (primary) current to reverse,

which in turn pulls current out of the output filter capacitor

and causes the flyback transformer to operate in the for-

ward mode. The low impedance presented by the trans-

former secondary in the forward mode dumps current into

the secondary output, charging up the secondary capac-

itor and bringing SECFB back into regulation. The SECFB

feedback loop does not improve secondary output accu-

racy in normal flyback mode, where the main (primary)

output is heavily loaded. In this mode, secondary output

accuracy is determined, as usual, by the secondary recti-

fier drop, turns ratio, and accuracy of the main output

voltage. So, a linear post-regulator may still be needed in

order to meet tight output accuracy specifications.

The secondary output voltage-regulation point is deter-

mined by an external resistor divider at SECFB. For nega-

tive output voltages, the SECFB comparator is referenced

to GND (MAX799); for positive output voltages, SECFB

regulates at the 2.505V reference (MAX796). As a result,

output resistor divider connections and design equations

for the two device types differ slightly (Figure 9).

Ordinarily, the secondary regulation point is set 5% to

10% below the voltage normally produced by the flyback

effect. For example, if the 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, caus-

ing an unnecessary increase in supply current and output

noise. In negative-output (MAX799) 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 MAX799

circuits.

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 20mV to 100mV. The main output

capacitor 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 capaci-

tance. With no SS capacitor connected, maximum cur-

rent limit is reached within 10µs.

Shutdown

Shutdown mode (

SHDN = 0V) reduces the V+ supply

current to typically 1µA. 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.

MAX796

MAX797

MAX799

MAX874

CSL

CSH

GND

VREF2 >>VREF

(4.096V)

MAIN

OUTPUT

VOUT = VREF - (VREF2 - VREF) (–––)

V+

Figure 7. Output Voltage Less than 2.5V

MAX796

MAX797

MAX799

CSL

CSH

0.01

μF

0.01

μF

GND

OUTPUT

(8V AS

SHOWN)

VOUT

RSENSE

DIVIDER IMPEDANCE

≤ 5kΩ

(EACH LEG)

= VREF (1 + –––)

V+

2.43k

1.1k

2.43k

1.1k

Figure 8. Adjusting the Output Voltage to Greater than 6V

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