MAX8792 Maxim Integrated Products, MAX8792 Datasheet - Page 21

• Manufacturer Part Number: MAX8792
• Description: Single Quick-PWM Step-Down Controller
• Manufacturer: Maxim Integrated Products

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Including an additional resistor between ILIM and the
output automatically creates a current-limit threshold that
folds back as the output voltage drops (see Figure 7).
The foldback current limit helps limit the inductor cur-
rent under fault conditions, but must be carefully
designed in order to provide reliable performance
under normal conditions. The current-limit threshold
must not be set too low, or the controller will not reliably
power up. To ensure the controller powers up properly,
the minimum current-limit threshold (when V
must always be greater than the maximum load during
startup (which at least consists of leakage currents),
plus the maximum current required to charge the out-
put capacitors:
The output filter capacitor must have low enough effec-
tive series resistance (ESR) to meet output ripple and
load-transient requirements. Additionally, the ESR
impacts stability requirements. Capacitors with a high
ESR value (polymers/tantalums) will not need additional
external compensation components.
Figure 7. Standard Application with Foldback Current-Limit Protection
I
START
LO
AGND
GND/OPEN/REF/V
HI
= C
5V BIAS
SUPPLY
OUT
ON
______________________________________________________________________________________
97.6kΩ
Output Capacitor Selection
R3
AGND
AGND
AGND
OFF
x 1mV/µs + I
CC
1000pF
54.9kΩ
1μF
C2
C3
49.9kΩ
Foldback Current Limit
100kΩ
R2
1μF
R10
C1
R1
PWR
LOAD(START)
13
14
12
10
11
2
1
Controller with Dynamic REFIN
PGOOD
EN
SKIP
REF
REFIN
V
V
DD
CC
Single Quick-PWM Step-Down
AGND
OUT
GND
MAX8792
= 0V)
PWR
(EP)
TON
ILIM
BST
DH
LX
DL
FB
7
9
6
5
4
3
8
In core and chipset converters and other applications
where the output is subject to large load transients, the
output capacitor’s size typically depends on how much
ESR is needed to prevent the output from dipping too
low under a load transient. Ignoring the sag due to
finite capacitance:
In low-power applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR. The maximum ESR to meet ripple require-
ments is:
where f
100kΩ
R
CBST
0.1μF
TON
R8
100kΩ
R5
100kΩ
AGND
PWR
100kΩ
SW
R4
R
is the switching frequency.
ESR
(
R
REF
ESR
≤
L1
⎡
⎢
⎢
⎣
(
+
V
IN
R
PCB
−
V f
IN SW
V
SEE TABLE 1 FOR COMPONENT SELECTION.
OUT
)
≤
Δ
PWR
C
C
)
PWR
L
IN
OUT
I
V
LOAD MAX
OUT
V
STEP
4V TO 12V
(
⎤
⎥
⎥
⎦
1.50V 10A
INPUT
1.05V 7A
OUTPUT
V
RIPPLE
)
21