MAX8792ETD+T Maxim Integrated Products, MAX8792ETD+T Datasheet - Page 22

• Manufacturer Part Number: MAX8792ETD+T
• Description: IC PWM CONTROLLER 14TDFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8792ETD+T

• Applications: PWM Controller
• Voltage - Input: 2 ~ 26 V
• Current - Supply: 700µA
• Operating Temperature: -40°C ~ 80°C
• Mounting Type: Surface Mount
• Package / Case: 14-TDFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Single Quick-PWM Step-Down
Controller with Dynamic REFIN
With most chemistries (polymer, tantalum, aluminum
electrolytic), the actual capacitance value required
relates to the physical size needed to achieve low ESR
and the chemistry limits of the selected capacitor tech-
nology. Ceramic capacitors provide low ESR, but the
capacitance and voltage rating (after derating) are
determined by the capacity needed to prevent V
and V
sients. Generally, once enough capacitance is added
to meet the overshoot requirement, undershoot at the
rising load edge is no longer a problem (see the V
and V
tion). Thus, the output capacitor selection requires
carefully balancing capacitor chemistry limitations
(capacitance vs. ESR vs. voltage rating) and cost.
For Quick-PWM controllers, stability is determined by
the in-phase feedback ripple relative to the switching
frequency, which is typically dominated by the output
ESR. The boundary of instability is given by the following
equation:
where C
total equivalent-series resistance of the output capaci-
tors, R
the output capacitors and feedback sense point, and
R
pled current-sense compensation (see Figure 10).
Figure 9. Remote-Sense Compensation for Stability and Noise Immunity
22
COMP
______________________________________________________________________________________
SOAR
SOAR
PCB
is the effective resistance of the DC- or AC-cou-
OUT
Output Capacitor Stability Considerations
is the parasitic board resistance between
equations in the Transient Response sec-
from causing problems during load tran-
is the total output capacitance, R
R
AGND
EFF
f
MAX8792
SW
=
π
GND
R
ESR
≥
TON
BST
DH
DL
LX
FB
2
PWR
π
+
R
R
EFF OUT
PCB
1
C
+
R
COMP
PWR
STABILITY REQUIREMENT
FEEDBACK RIPPLE IN PHASE WITH INDUCTOR CURRENT
L1
R
ESR
C
PWR
IN
ESR
C
OUT
≥
is the
2f
SAG
SAG
1
SW
PWR
C
AND R
OUT
INPUT
R
100Ω
COMP
COMP
For a standard 300kHz application, the effective zero
frequency must be well below 95kHz, preferably below
50kHz. With these frequency requirements, standard
tantalum and polymer capacitors already commonly
used have typical ESR zero frequencies below 50kHz,
allowing the stability requirements to be achieved with-
out any additional current-sense compensation. In the
standard application circuit (Figure 1), the ESR needed
to support a 15mV
5mΩ. Two 330µF, 9mΩ polymer capacitors in parallel
provide 4.5mΩ (max) ESR and 1 / (2π x 330µF x 9mΩ)
= 53kHz ESR zero frequency.
Figure 8. Standard Application with Output Polymer or Tantalum
AGND
C
COMP
PWR
C
LOAD
MAX8792
≥
GND
f
PCB PARASITIC RESISTANCE
SENSE RESISTANCE FOR EVALUATION
SW
1
TON
BST
DH
DL
LX
FB
PWR
P-P
OUTPUT VOLTAGE REMOTELY
SENSED NEAR POINT OF LOAD
PWR
ripple is 15mV / (10A x 0.3) =
OUTPUT
L1
STABILITY REQUIREMENT
C
PWR
R
IN
ESR
C
OUT
≥
PWR
C
OUT
2f
1
SW
INPUT
OUTPUT