![]() |
Electronic Components Datasheet Search |
|
ADP1871 Datasheet(PDF) 23 Page - Analog Devices |
|
|
ADP1871 Datasheet(HTML) 23 Page - Analog Devices |
23 / 44 page ![]() ADP1870/ADP1871 Rev. 0 | Page 23 of 44 C R (TRIMMED) VREG tON VIN I To illustrate this feature more clearly, this section describes one such load transient event—a positive load step—in detail. During load transient events, the high-side driver output pulse width stays relatively consistent from cycle to cycle; however, the off-time (DRVL on-time) dynamically adjusts according to the instantaneous changes in the external conditions mentioned. SW INFORMATION When a positive load step occurs, the error amplifier (out of phase of the output, VOUT) produces new voltage information at its output (COMP). In addition, the current-sense amplifier senses new inductor current information during this positive load transient event. The error amplifier’s output voltage reaction is compared with the new inductor current information that sets the start of the next switching cycle. Because current information is produced from valley current sensing, it is sensed at the down ramp of the inductor current, whereas the voltage loop information is sensed through the counter action upswing of the error amplifier’s output (COMP). Figure 77. Constant On-Time Time The constant on-time (tON) is not strictly “constant” because it varies with VIN and VOUT. However, this variation occurs in such a way as to keep the switching frequency virtually independent of VIN and VOUT. The tON timer uses a feedforward technique, applied to the constant on-time control loop, making it a pseudo-fixed frequency to a first order. Second-order effects, such as dc losses in the external power MOSFETs (see the Efficiency Consideration section), cause some variation in frequency vs. load current and line voltage. These effects are shown in Figure 23 to Figure 34. The variations in frequency are much reduced compared with the variations generated when the feedforward technique is not utilized. The result is a convergence of these two signals (see Figure 78), which allows an instantaneous increase in switching frequency during the positive load transient event. In summary, a positive load step causes VOUT to transient down, which causes COMP to transient up and therefore shortens the off-time. This resulting increase in frequency during a positive load transient helps to quickly bring VOUT back up in value and within the regulation window. The feedforward technique establishes the following relationship: K f SW 1 = where fSW is the controller switching frequency (300 kHz, 600 kHz, and 1.0 MHz). Similarly, a negative load step causes the off-time to lengthen in response to VOUT rising. This effectively increases the inductor demagnetizing phase, helping to bring VOUT within regulation. In this case, the switching frequency decreases, or experiences a foldback, to help facilitate output voltage recovery. The tON timer senses VIN and VOUT to minimize frequency variation as previously explained. This provides a pseudo-fixed frequency as explained in the Pseudo-Fixed Frequency section. To allow headroom for VIN and VOUT sensing, adhere to the following equations: Because the ADP1870/ADP1871 has the ability to respond rapidly to sudden changes in load demand, the recovery period in which the output voltage settles back to its original steady state operating point is much quicker than it would be for a fixed-frequency equivalent. Therefore, using a pseudo-fixed frequency results in significantly better load transient performance than using a fixed frequency. VREG ≥ VIN/8 + 1.5 VREG ≥ VOUT/4 For typical applications where VREG is 5 V, these equations are not relevant; however, for lower VREG inputs, care may be required. VALLEY TRIP POINTS LOAD CURRENT DEMAND ERROR AMP OUTPUT PWM OUTPUT fSW > fSW CS AMP OUTPUT PSEUDO-FIXED FREQUENCY The ADP1870/ADP1871 employ a constant on-time control scheme. During steady state operation, the switching frequency stays relatively constant, or pseudo-fixed. This is due to the one- shot tON timer that produces a high-side PWM pulse with a “fixed” duration, given that external conditions such as input voltage, output voltage, and load current are also at steady state. During load transients, the frequency momentarily changes for the duration of the transient event so that the output comes back within regulation more quickly than if the frequency were fixed or if it were to remain unchanged. After the transient event is complete, the frequency returns to a pseudo-fixed frequency value to a first order. Figure 78. Load Transient Response Operation |
Similar Part No. - ADP1871 |
|
Similar Description - ADP1871 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |