MAX15014_07 Datasheet, PDF (16/26 Pages) Maxim Integrated Products – 1A, 4.5V to 40V Input Buck Converters with 50mA Auxiliary LDO Regulators

1A, 4.5V to 40V Input Buck Converters with
50mA Auxiliary LDO Regulators
Soft-Start and Reference (SS)
SS is the 1.235V reference bypass connection for the
MAX15014–MAX15017 and also controls the soft-start
period. At startup, after input voltage is applied at IN_SW,
IN_LDO and the UVLO thresholds are reached, the
device enters soft-start. During soft-start, 14µA is sourced
into the capacitor (C_SS) connected from SS to SGND
causing the reference voltage to ramp up slowly. When
V_SS reaches 1.244V, the output becomes fully active. Set
the soft-start time (t_SS) using following equation:
t_SS
=
V_SS × C_SS
I_SS
where V_SS = soft-start reference voltage = 1.235V (typ),
I_SS = soft-start current = 14 x 10^-6A (typ), t_SS is in sec-
onds and C_SS is in Farads.
Internal Charge Pump
(MAX15015/MAX15016)
The MAX15015/MAX15016 feature an internal charge
pump to enhance the turn-on of the internal MOSFET,
allowing for operation with input voltages down to 4.5V.
Connect a flying capacitor (C_F) between C+ and C-, a
boost diode from C+ to BST, as well as a bootstrap
capacitor (C_BST) between BST and LX to provide the
gate drive voltage for the high-side n-channel DMOS
switch. During the on-time, the flying capacitor is
charged to V_DVREG. During the off-time, the positive ter-
minal of the flying capacitor (C+) is pumped to two times
V_DVREG and charge is dumped onto C_BST to provide
twice the regulator voltage across the high-side DMOS
driver. Use a ceramic capacitor of at least 0.1µF for
C_BST and C_F located as close as possible to the device.
Gate Drive Supply (DVREG)
DVREG is the supply input for the internal high-side
MOSFET driver. The power for DVREG is derived from
the output of the internal regulator (REG). Connect
DVREG to REG externally. To filter the switching noise,
the use of an RC filter (1Ω and 0.47µF) from REG to
DVREG is recommended. In the MAX15015/MAX15016,
the high-side drive supply is generated using the inter-
nal charge pump along with the bootstrap diode and
capacitor. In the MAX15014/MAX15017, the high-side
MOSFET driver supply is generated using only the boot-
strap diode and capacitor.
Error Amplifier
The output of the internal error amplifier (COMP) is avail-
able for frequency compensation (see the Compensation
Design section). The inverting input is FB, the noninverting
input SS, and the output COMP. The error amplifier has an
80dB open-loop gain and a 1.8MHz GBW product. See
the Typical Operating Characteristics for the Gain and
Phase vs. Frequency graph.
Oscillator/Synchronization Input (SYNC)
With SYNC connected to SGND, the MAX15014–
MAX15017 use their internal oscillator and switch at a
fixed frequency of 135kHz and 500kHz. The MAX15014/
MAX15016 are the 135kHz options and MAX15015/
MAX15017 are the 500kHz options. For external syn-
chronization, drive SYNC with an external clock from
400kHz to 600kHz (MAX15015/MAX15017) or 100kHz
to 200kHz (MAX15014/MAX15016). When driven with
an external clock, the device synchronizes to the rising
edge of SYNC.
PWM Comparator/Voltage Feed-Forward
An internal ramp generator clocked by the internal
oscillator is compared against the output of the error
amplifier to generate the PWM signal. The maximum
amplitude of the ramp (V_RAMP) automatically adjusts to
compensate for input voltage and oscillator frequency
changes. This causes the V_{IN_SW} / V_RAMP to be a
constant 10V/V across the input voltage range of 4.5V to
40V (MAX15015/MAX15016) or 7.5V to 40V (MAX15014/
MAX15017) and the SYNC frequency range of 400kHz
to 600kHz (MAX15015/MAX15017) or 100kHz to 200kHz
(MAX15014/MAX15016).
Output Short-Circuit Protection
(Hiccup Mode)
The MAX15014–MAX15017 protect against an output
short circuit by utilizing hiccup-mode protection. In hic-
cup mode, a series of sequential cycle-by-cycle
current-limit events cause the part to shut down and
restart with a soft-start sequence. This allows the
device to operate with a continuous output short circuit.
During normal operation, the current is monitored at the
drain of the internal power MOSFET. When the current
limit is exceeded, the internal power MOSFET turns off
until the next on-cycle and a counter increments. If the
counter counts seven consecutive current-limit events,
the device discharges the soft-start capacitor and
shuts down for 512 clock periods before restarting with
a soft-start sequence. Each time the power MOSFET
turns on and the device does not exceed the current
limit, the counter is reset.
LDO Regulator
The LDO regulator operates over an input voltage from
5V to 40V, and can be enabled independently of the
DC-DC converter section. Its quiescent current is as
low as 47µA with a load current of 100µA. All devices
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