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MAX1515ETG+T

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
MAX1515
Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
______________________________________________________________________________________ 13
MAX1515
LX
GND
V
CC
FB
COMP
PGND
IN
REFOUT
PGOOD
V
BIAS
(3.0V TO 3.6V)
V
CC
(DDR MODE)
V
IN
(1.3V TO 3.6V)
V
OUT
= V
TT
= V
DDC
/ 2
V
REFOUT
= V
TTR
V
DDQ
(2.5V OR 1.8V)
MODE
FBSEL1
BST
V
DD
SHDN
SKIP
FBSEL0
SS
REFIN
REF
TOFF
R1
10Ω
R2
100kΩ
R
TOFF
110kΩ
R
SS
(OPTIONAL)
C
SS
0.01μF
C
BST
0.01μF
PWM MODE
SKIP MODE
ON
OFF
R3
10kΩ
1%
R4
10kΩ
1%
SEE TABLE 1 FOR COMPONENT SPECIFICATIONS
C
VDD
1μF
C
IN
33μF
C
OUT
220μF
C
REFOUT
0.47μF
L
1.2μH
C
VCC
1μF
C
REFIN
0.01μF
C
COMP
470pF
C
REF
1μF
ERR
MAX1515
LX
SNK/SRC
THRESHOLD
SRC
SNK
IN
PGND
FB
FB GAIN
SINK/
SOURCE
LOGIC
SS
SOFT-
START
PWM
LOGIC
COMP
TOFF
TIMER
CURRENT
SENSE
ZX(+)
CURRENT
SENSE (+/-)
ZX(-)
GND
BST
SHDN
MODE
FBSEL0FBSEL1
FBSEL
DECODE
SKIP
V
DD
L-SIDE
DRIVER
H-SIDE
DRIVER
PGOOD
PGOOD
LOGIC
V
DD
REF BUF
REFIN
REFOUT
REF
REF
1.1V
V
CC
MODE
G
m
Figure 1. Standard Application Circuit
Figure 2. Functional Diagram
MAX1515
Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
14 ______________________________________________________________________________________
+3.3V Bias Supply (V
CC
and V
DD
)
The MAX1515 requires a 3.3V bias supply for its inter-
nal circuitry. Typically, this 3.3V bias supply is the note-
book’s 95%-efficient, 3.3V system supply. The 3.3V
bias supply must provide V
CC
(PWM controller) and
V
DD
(gate-drive and reference buffer power), so the
maximum current drawn is:
I
BIAS
= I
CC
+ I
REFOUT
+ f
SW
(Q
G(LOW)
+ Q
G(HIGH)
)
where I
CC
is 450µA (typ), f
SW
is the switching frequen-
cy, and Q
G(LOW)
and Q
G(HIGH)
are the internal MOS-
FET total gate charge of approximately 1nC.
The input supply (V
IN
) and 3.3V bias inputs (V
CC
and
V
DD
) can be connected together if the input source is a
fixed 3.0V to 3.6V supply. If the 3.3V bias supply pow-
ers up prior to the input supply, the enable signal
(SHDN going from low to high) must be delayed until
the input voltage is present to ensure startup.
Current Limit
The MAX1515 provides peak current limiting to protect
the MOSFETs during source/sink overload and short
circuit. During source mode the controller switches the
high-side MOSFET off when the inductor current
exceeds 4.2A. Use the following equation to calculate
the maximum source current:
where I
SOURCE_MAX
is the maximum source current,
I
LIMIT_P
is the source inductor current limit (4.2A typ),
and t
OFF
is the fixed off-time. For typical operating con-
ditions and component selection, this results in a maxi-
mum source current of 3.7A.
In sink mode, the MAX1515 does not issue an off-time
until the inductor current is above -3.2A. Use the follow-
ing equation to calculate the maximum sink current:
where I
SINK_MAX
is the maximum sink current, I
LIMIT_N
is the sink inductor current limit (-3.0A typ), t
DLY
is the
current-limit propagation delay of approximately 500ns,
and t
OFF
is the fixed off-time. For typical operating con-
ditions and component selection, this results in a maxi-
mum sink current of -2.5A.
Soft-Start Current Limit
Soft-start allows a gradual increase of the internal cur-
rent limit to reduce input surge currents at startup and at
exit from shutdown. A timing capacitor, C
SS
, placed
from SS to GND sets the rate at which the internal cur-
rent limit is changed. Upon power-up, when the device
comes out of undervoltage lockout (2.6V typ) or after the
SHDN pin is pulled high, a 5µA (typ) constant-current
source charges the soft-start capacitor and the voltage
on SS increases. When the voltage on SS is less than
approximately 0.7V, the current limit is set to zero. As
the voltage increases from 0.7V to approximately 1.8V,
the current limit is adjusted from 0 to the current-limit
threshold (see the Electrical Characteristics). The volt-
age across the soft-start capacitor changes with time
according to the equation:
where I
SS(SRC)
is the soft-start source current from the
Electrical Characteristics.
The time when full current limit is available is given by:
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
where I
LIMIT_P
is the positive current threshold from the
Electrical Characteristics. The constant-current source
stops charging once the voltage across the soft-start
capacitor reaches 1.8V (Figure 3).
Adjustable Positive Current Limit
The MAX1515 has internal current-limit circuitry that
limits the maximum current through the NMOS to 4.2A.
For applications that require a lower current limit, the
maximum current limit can be reduced by placing a
resistor (R
SS
) from SS to GND. The time constant for
the soft-start current limit is R
SS
x C
SS
.
where I
LIMIT
is the desired reduced current limit, and
I
LIMIT_P
and I
SS(SRC)
are taken from the Electrical
Characteristics.
R
VI
I
VI
SS
REF LIMIT
LIMIT P
SS SRC
=
×
+
⎛
⎝
⎜
⎞
⎠
⎟
. /
_
()
07
SSI
VV
V
I
LIMIT
SS
REF
LIMIT P
=×
− 07.
_
t
CV
I
SS
SS SRC
=
× 18.
()
V
It
C
SS
SS SRC
SS
=
×
()
II
Vt V V t
L
SINK MAX
LIMIT N
OUT OFF IN OUT DLY
_
_
=+
()
−−2
2
II
Vt
L
SOURCE MAX LIMIT P
OUT OFF
__
=
×
×
−
2
MAX1515
Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
______________________________________________________________________________________ 15
Short-Circuit/Overload Protection
The MAX1515 can sustain a constant short circuit or
overload. Under a source-mode short-circuit or over-
load condition, when V
FB
< 0.3 x V
TARGET
, the
MAX1515 uses an extended off-time to control the cur-
rent. Operation during a short circuit or overload is sim-
ilar to forced-PWM mode except the off-time is 4 x t
OFF.
At the end of each off-time, the high-side NMOS switch
turns on and remains on until the output is in regulation
or the current through the switch increases to the maxi-
mum current limit. When the high-side NMOS switch
turns off, it remains off for four times the programmed
off-time (t
OFF
), and the low-side NMOS synchronous
switch turns on. Since either NMOS switch is always on,
the inductor current is continuous. The RMS inductor
current during a short circuit remains below the maxi-
mum current-limit threshold. The MAX1515 operates
using the extended off-time until the short circuit or
overload is removed and V
FB
> 0.3 x V
TARGET
.
Prolonged short circuit or overload can result in thermal
shutdown.
Summing Comparator
Three signals are added together at the input of the
summing comparator (Figure 2): an output-voltage
error signal relative to the reference voltage, an inte-
grated output-voltage error-correction signal, and the
sensed high-side NMOS switch current. The integrated
error signal is provided by a transconductance amplifi-
er with an external capacitor at COMP. This integrator
provides high DC accuracy without the need for a high-
gain amplifier. Connecting a capacitor at COMP modi-
fies the overall loop response (see the Integrator
Amplifier section).
Integrator Amplifier
The MAX1515 includes an internal transconductance
amplifier that improves the output DC accuracy.
A capacitor, C
COMP
, from COMP to V
CC
compensates
the transconductance amplifier. For stability, choose
C
COMP
= 470pF.
Modes of Operation (MODE)
Use MODE to configure the MAX1515 for DDR mode
(MODE = V
CC
) or non-DDR mode (MODE = GND). In
DDR mode, the MAX1515 can sink current even while
SKIP is low (see the Pulse Skipping (Source Mode) and
Pulse Skipping (Sink Mode) sections). Also, DDR mode
enables the REFOUT buffer, providing a buffered out-
put of the REFIN voltage. In non-DDR mode, the
MAX1515 can only source current when SKIP is low.
The REFOUT buffer is also disabled in non-DDR mode.
Light-Load Operation (
SKIP
)
The MAX1515 includes a pulse-skipping mode that
reduces current consumption during light loads. To con-
figure the MAX1515 for pulse-skipping mode, connect
SKIP to GND. Forced-PWM mode keeps the switching
frequency relatively constant and is desirable in appli-
cations that must always keep the frequency of con-
ducted and radiated emissions in a narrow band. Visit
Maxim’s website (www.maxim-ic.com) for more informa-
tion on how to control electromagnetic interference
(EMI). Pulse-skipping mode has a dynamic switching
frequency under light loads and is desirable in applica-
tions that require high efficiency at light loads.
Forced-PWM Mode
Connect SKIP to V
CC
to force the MAX1515 to operate
in low-noise, constant-off-time PWM mode. Constant
off-time PWM architecture provides a relatively constant
switching frequency (see the Frequency Variation with
Output Current section). A single resistor (R
TOFF
) sets
the high-side NMOS power-switch off-time that results
in a switching frequency up to 1MHz, allowing perfor-
mance trade-offs in efficiency, switching noise, compo-
nent size, and cost.
Forced-PWM mode regulates the output voltage by
increasing the high-side NMOS switch on-time to
increase the amount of energy transferred to the load
per cycle. At the end of each off-time, the high-side
NMOS switch turns on and remains on until the output is
in regulation or the current through the switch reaches
the 4.2A current limit. When the high-side NMOS switch
turns off, it remains off for the programmed off-time
(t
OFF
), and the low-side NMOS synchronous switch
turns on. The low-side NMOS switch remains on until the
end of t
OFF
. Since either NMOS switch is always on in
PWM mode, the inductor current is continuous.
1.8V
0.7V
I
LIMIT_P
V
SS
(V)
I
LIMIT
(A)
SHDN
Figure 3. Soft-Start Current Limit
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MAX1515ETG+T

Mfr. #:
Buy MAX1515ETG+T
Manufacturer:
Maxim Integrated
Description:
Switching Voltage Regulators 3A 1MHz Step-Down DDR Regulator
Lifecycle:
New from this manufacturer.
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