MAX1515 Datasheet, PDF (16/24 Pages) Maxim Integrated Products – Low-Voltage, Internal Switch, Step-Down/DDR Regulator

Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
Table 3. Modes of Operation
SHDN
Low
Low
High
MODE
PIN
Low
High
Low
SKIP
X
X
Low
REFOUT
BUFFER
Off, High-Z
On
Off, High-Z
STEP-DOWN
REGULATOR MODE
Off
Off
On, non-DDR mode.
FB regulates to preset voltage or 0.5V.
STEP-DOWN
REGULATOR CURRENT
Off
Off
Source only.
Pulse-skipping mode.
High
Low
High Off, High-Z
On, non-DDR mode.
FB regulates to preset voltage or 0.5V.
Source/sink.
Forced-PWM mode.
High
High
Low
On
On, DDR mode.
FB regulates to REFIN.
Source/sink.
Pulse-skipping mode.
High
High
High
On
On, DDR mode.
FB regulates to REFIN.
Source/sink.
Forced-PWM mode.
X = Don’t care.
Pulse Skipping (Source Mode)
Connect SKIP to GND to allow the MAX1515 to auto-
matically switch between high-efficiency pulse-skipping
mode under light loads and PWM mode under heavy
loads. The transition from PWM mode to pulse-skipping
mode occurs when the load current is half the pulse-
skipping mode current threshold (800mA typ).
In pulse-skipping mode, the switching frequency is
reduced to increase efficiency. The inductor current is
discontinuous in this mode, and the MAX1515 only initi-
ates an LX switching cycle when V_FB < V_REFIN. When
V_FB falls below V_REFIN, the high-side NMOS switch
turns on and remains on until output is in regulation and
the current through the switch increases to the positive
pulse-skipping-mode current threshold (I_{SKIP_P}) of
800mA. When the high-side NMOS switch turns off, the
low-side NMOS synchronous switch turns on and
remains on until the current through the switch decreas-
es to the zero-cross-current threshold of 200mA.
Pulse Skipping (Sink Mode)
When pulse-skipping operation is selected (SKIP =
GND) while in DDR mode (MODE = V_CC), the
MAX1515’s source/sink controller switches operating
modes when the output voltage crosses either hys-
teretic sink/source thresholds (V_REFIN ±25mV). In
pulse-skipping source mode, the MAX1515 regulates
the valley of the output ripple voltage (see the Pulse
Skipping (Source Mode) section). When the output volt-
age rises above the sink-mode threshold, the MAX1515
enters sink mode. The MAX1515 begins each sink-
mode cycle by turning on the low-side NMOS. The low-
side NMOS remains on until the off-time (t_OFF). After the
low-side NMOS turns off, the high-side NMOS turns on
Table 4. Output-Voltage Programming
FBSEL0
GND
GND
V_CC
V_CC
FBSEL1
GND
V_CC
GND
V_CC
OUTPUT
VOLTAGE
Adjustable
V_FB = V_REFIN
1.5V
1.8V
2.5V
and remains on until the current through the switch
reaches the zero-cross-current threshold of -350mA. As
long as the output voltage remains below the feedback
threshold, the controller remains in the high-impedance
state. Under light-load conditions, this allows the sink-
mode controller to automatically skip pulses. Under
heavy-load conditions, the output voltage remains
above the feedback threshold, forcing the sink-mode
controller to emulate typical forced-PWM operation.
The pulse-skipping current threshold allows the sink-
mode control scheme to automatically switch between
pulse-skipping PFM and nonskipping PWM operation.
This mechanism forces the boundary between continu-
ous and discontinuous inductor-current operation to be
half the negative pulse-skipping current threshold.
Output Voltage in Non-DDR Mode
In non-DDR mode (MODE = GND and V_REFIN = V_REF),
the output of the MAX1515 is selectable between one
of three preset output voltages: 2.5V, 1.8V, and 1.5V.
For a preset output voltage, connect FB to the output
voltage and connect FBSEL0 and FBSEL1 as indicated
in Table 4. For an adjustable output voltage, connect
FBSEL0 and FBSEL1 to GND and connect REFIN to a
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