Datasheet LT1961 (Linear Technology) - 7

LT1961
U U W U APPLICATIONS INFORMATION FB RESISTOR NETWORK
defines the pole frequency of the output stage, an X7R or X5R type ceramic, which have good temperature stability, The suggested resistance (R2) from FB to ground is 10k is recommended. 1%. This reduces the contribution of FB input bias current to output voltage to less than 0.2%. The formula for the Tantalum capacitors are usually chosen for their bulk resistor (R1) from VOUT to FB is: capacitance properties, useful in high transient load appli- cations. ESR rather than absolute value defines output R2 V ( . ) OUT − 1 2 ripple at 1.25MHz. Values in the 22μF to 100μF range are R1= generally needed to minimize ESR and meet ripple current 1.2 − R2 0 ( .2 A μ ) ratings. Care should be taken to ensure the ripple ratings are not exceeded.
Table 1. Surface Mount Solid Tantalum Capacitor ESR and
LT1961 VSW
Ripple Current
OUTPUT ERROR
E Case Size ESR (Max,
Ω
) Ripple Current (A)
AMPLIFIER AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 + 1.2V R1 AVX TAJ 0.7 to 0.9 0.4 FB +
D Case Size
– AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 R2 10k
C Case Size
1961 F02 AVX TPS 0.2 (typ) 0.5 (typ) VC GND
Figure 2. Feedback Network INPUT CAPACITOR
Unlike the output capacitor, RMS ripple current in the
OUTPUT CAPACITOR
input capacitor is normally low enough that ripple current Step-up regulators supply current to the output in pulses. rating is not an issue. The current waveform is triangular, The rise and fall times of these pulses are very fast. The with an RMS value given by: output capacitor is required to reduce the voltage ripple this causes. The RMS ripple current can be calculated 0 2 . 9 V V from: ( −V ( ) IN OUT IN IRIPPLE RMS ( ) = L f V ( )( )( ) OUT I I V V V ( )/ RIPPLE RMS OUT OUT IN IN ( ) = − At higher switching frequency, the energy storage require- The LT1961 will operate with both ceramic and tantalum ment of the input capacitor is reduced so values in the output capacitors. Ceramic capacitors are generally cho- range of 1μF to 4.7μF are suitable for most applications. sen for their small size, very low ESR (effective series Y5V or similar type ceramics can be used since the resistance), and good high frequency operation, reducing absolute value of capacitance is less important and has no output ripple voltage. Their low ESR removes a useful zero significant effect on loop stability. If operation is required in the loop frequency response, common to tantalum close to the minimum input voltage required by either the capacitors. To compensate for this, the V output or the LT1961, a larger value may be necessary. C loop compen- sation pole frequency must typically be reduced by a factor This is to prevent excessive ripple causing dips below the of 10. Typical ceramic output capacitors are in the 1μF to minimum operating voltage resulting in erratic operation. 10μF range. Since the absolute value of capacitance 1961fa 7