HM1084 datasheet(8/9 Pages) HMSEMI

8 / 9 page

Rev. 04

It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output

voltage by reducing the overall ESR.

Output capacitance can be increased indefinitely to improve transient response and stability.

RIPPLE REJECTION

Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 1 and Figure 2). The

HM1084

exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than VREF, the ripple rejection decreases as

function of adjustment gain: (1+R1/R2) or VO/VREF. Therefore a 5V adjustment decreases ripple rejection by a factor of

four (−12dB); Output ripple increases as adjustment voltage increases.

However, the adjustable version allows this degradation of ripple rejection to be compensated. The adjust terminal can

be bypassed to ground with a capacitor (CADJ). The impedance of the CADJ should be equal to or less than R1 at the

desired ripple frequency. This bypass capacitor prevents ripple from being amplified as the output voltage is increased.

LOAD REGULATION

The

HM1084 regulates the voltage that appears between its output and ground pins, or between its output and adjust pins.

In some cases, line resistances can introduce errors to the voltage across the load. To obtain the best load regulation, a

few precautions are needed.

Figure 3

shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. VLOAD is less

than the VOUT by the sum of the voltage drops along the line resistances. In this case, the load regulation seen at the

RLOAD would be degraded from the data sheet specification. To improve this, the load should be tied directly to the output

terminal on the positive side and directly tied to the ground terminal on the negative side.

FIGURE 3. Typical Application using Fixed Output Regulator

When the adjustable regulator is used (Figure 4), the best performance is obtained with the positive side of the resistor

R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops from appearing

effectively in series with the reference and degrading regulation. For example, a 5V regulator with 0.05

resistance

between the regulator and load will have a load regulation due to line resistance of 0.05

x IL. If R1 (=125

) is

connected near the load the effective line resistance will be 0.05

(1 + R2/R1) or in this case, it is 4 times worse. In

addition, the ground side of the resistor R2 can be returned near the ground of the load to provide remote ground

sensing and improve load regulation.

FIGURE 4. Best Load Regulation using Adjustable Output Regulator

PROTECTION DIODES

Under normal operation, the

HM1084 regulator does not need any protection diode. With the adjustable device, the

internal resistance between the adjustment and output terminals limits the current. No diode is needed to divert the

current around the regulator even with a capacitor on the adjustment terminal. The adjust pin can take a transient signal

of ±25V with respect to the output voltage without damaging the device.

Shenzhen H&M Semiconductor Co.Ltd

http：//www.hmsemi.com

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