MAX1515 Datasheet, PDF (17/24 Pages) Maxim Integrated Products – Low-Voltage, Internal Switch, Step-Down/DDR Regulator

Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
resistive divider between REF and ground (Figure 5).
Regulation is maintained for adjustable output voltages
when V_FB = V_REFIN. Use 100kΩ for R_B. R_A is given by
the equation:
V_FB =
V_REF
⎛
⎝⎜
R_B
R_A +
R_B
⎞
⎠⎟
where V_REF = 1.1V.
The preset output voltages use an internally trimmed
resistor-divider network that sets the output voltage to
the correct level when REFIN is connected to REF.
Connecting REFIN to other voltage levels while using
the preset voltage modes results in a ratiometrically
scaled output voltage.
Output Voltage in DDR Mode
In DDR mode (MODE = V_CC), the MAX1515 regulates
FB to the voltage set at REFIN. For DDR applications,
the termination supply must track to exactly half the
memory supply voltage. Figure 1 shows the MAX1515
configured for DDR applications.
Reference Buffer (REFOUT)
A unity-gain amplifier provides a buffered output for the
reference input (V_REFIN) when MODE = V_CC. This
transconductance amplifier must be compensated with
a 0.47µF or greater ceramic capacitor. Larger capaci-
tor values decrease the amplifier’s bandwidth, thereby
increasing the response time to dynamic input-voltage
changes. The buffer allows this dynamic reference to
remain within ±20mV of the input voltage (V_REFIN) even
when loaded with ±5mA. The input voltage range of the
amplifier is 0.5V to 1.5V. The reference buffer shuts
down when MODE = GND.
Power-Good Output (PGOOD)
PGOOD is the open-drain output for a window compara-
tor that continuously monitors the output. PGOOD is
actively held low in shutdown and during soft-start. After
soft-start terminates, PGOOD becomes high impedance
as long as the respective output voltage is within ±10%
of the nominal regulation voltage. When the output volt-
age drops 10% below or rises 10% above the nominal
regulation voltage, the MAX1515 pulls the power-good
output (PGOOD) low by turning on the MOSFET (Figure
2). For logic-level output voltages, connect an external
pullup resistor between PGOOD and V_CC. A 100kΩ
resistor works well in most applications.
Thermal Shutdown
The MAX1515 features a thermal fault-protection circuit.
When the junction temperature rises above +165°C, a
thermal sensor shuts down the MAX1515 regardless of
LOAD
CURRENT
INDUCTOR
CURRENT
OUTPUT
VOLTAGE
LX
VOLTAGE
t_OFF
Figure 4. Sink-Mode Waveforms
L
REF
LX
R_A
MAX1515
PGND
REFIN
GND
R_B
FB
MODE
FBSEL0
FBSEL1
0
-2A
0
-2A
V_REFIN
V_IN
V_OUT
GND
V_OUT
C_OUT
Figure 5. Setting VOUT with a Resistive Voltage-Divider at REFIN
V_SHDN. The MAX1515 is reactivated after the junction
temperature cools to +150°C.
Thermal Resistance
Junction-to-ambient thermal resistance, θ_JA, is highly
dependent on the amount of copper area connected to
the exposed backside pad. Airflow over the board sig-
nificantly reduces θ_JA. For heatsinking purposes, evenly
distribute the copper area connected at the IC among
the high-current pins. Refer to the Maxim website
(www.maxim-ic.com) for QFN thermal considerations.
Power Dissipation
Power dissipation in the MAX1515 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to supply current in the control
section and average current used to charge and dis-
charge the gate capacitance of the internal switches
(i.e., switching losses—PSL) is approximately:
PSL = C x V_IN² x f_SW
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