Datasheet

MAX8792
Single Quick-PWM Step-Down
Controller with Dynamic REFIN
______________________________________________________________________________________ 21
Foldback Current Limit
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
OUT
= 0V)
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:
I
START
= C
OUT
x 1mV/μs + I
LOAD(START)
Output Capacitor Selection
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.
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
SW
is the switching frequency.
R
Vf L
VV V
V
ESR
IN SW
IN OUT OUT
RIPPLE
≤
−
()
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
RR
V
I
ESR PCB
STEP
LOAD MAX
+
()
≤
Δ
()
C1
1μF
EN
OFFON
V
DD
V
CC
C2
1μF
PGOOD
R10
100kΩ
REFIN
R2
54.9kΩ
BST
LX
CBST
0.1μF
L1
C
OUT
TON
R3
97.6kΩ
GND/OPEN/REF/V
DD
DH
DL
FB
HILO
REF
ILIM
REF
R
TON
100kΩ
R5
100kΩ
R8
100kΩ
R4
100kΩ
AGND
AGND
PWR
PWR
PWR
AGND
AGND
PWR
AGND
AGND
1
10
12
2
13
7
6
5
4
3
GND (EP)
14
SKIP
11
8
9
OUTPUT
1.50V 10A
1.05V 7A
INPUT
4V TO 12V
5V BIAS
SUPPLY
MAX8792
C
IN
PWR
C3
1000pF
R1
49.9kΩ
Figure 7. Standard Application with Foldback Current-Limit Protection
SEE TABLE 1 FOR COMPONENT SELECTION.