MP1474 – SYNCHRONOUS STEP-DOWN CONVERTER
MP1474 Rev. 1.0 www.MonolithicPower.com 10
9/26/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
Figure 3: Adjustable UVLO
Internal Soft-Start
The soft-start prevents the converter output
voltage from overshooting during startup. When
the chip starts, the internal circuitry generates a
soft-start voltage (V
SS
) that ramps up from 0V to
1.2V. When V
SS
is less than V
REF
, the error
amplifier uses V
SS
as the reference. When V
SS
exceeds V
REF
, the error amplifier uses V
REF
as
the reference. The SS time is internally set to
1.2ms.
Power Good Indicator
MP1474 has an open drain pin as the power-
good indicator (PG). Pull this up to VCC or
another external source through a 100k
resistor. When V
FB
exceeds 90% of V
REF
, PG
switches goes high with 0.4ms delay time. If V
FB
goes below 85% of V
REF
, an internal MOSFET
pulls the PG pin down to ground.
The internal circuit keeps the PG low once the
input supply exceeds 1.2V.
Over-Current-Protection and Hiccup
The MP1474 has a cycle-by-cycle over-current
limit when the inductor current peak value
exceeds the set current limit threshold.
Meanwhile, the output voltage drops until V
FB
is
below the Under-Voltage (UV) threshold—
typically 50% below the reference. Once UV is
triggered, the MP1474 enters hiccup mode to
periodically restart the part. This protection
mode is especially useful when the output is
dead-shorted to ground, and greatly reduces
the average short circuit current to alleviate
thermal issues and protect the regulator. The
MP1474 exits the hiccup mode once the over-
current condition is removed.
Thermal Shutdown
Thermal shutdown prevents the chip from
operating at exceedingly high temperatures.
When the silicon die reaches temperatures that
exceed 150°C, it shuts down the whole chip.
When the temperature drops below its lower
threshold, typically 130°C, the chip is enabled
again.
Floating Driver and Bootstrap Charging
An external bootstrap capacitor powers the
floating power MOSFET driver. This floating
driver has its own UVLO protection. This
UVLO’s rising threshold is 2.2V with a
hysteresis of 150mV. The bootstrap capacitor
voltage is regulated internally by V
IN
through
D1, M1, R3, C4, L1 and C2 (Figure 4). If (V
IN
-
V
SW
) exceeds 5V, U1 will regulate M1 to
maintain a 5V BST voltage across C4. A 20
resistor placed between SW and BST cap. is
strongly recommended to reduce SW spike
voltage.
V
IN
D1
5V
M1
U1
BST
C4
SW
L1
V
OUT
C2
R3
Figure 4: Internal Bootstrap Charging Circuit
Startup and Shutdown
If both V
IN
and V
EN
exceed their respective
thresholds, the chip starts. The reference block
starts first, generating stable reference voltage
and currents, and then the internal regulator is
enabled. The regulator provides a stable supply
for the remaining circuitries.
Three events can shut down the chip: V
EN
low,
V
IN
low, and thermal shutdown. During the
shutdown procedure, the signaling path is first
blocked to avoid any fault triggering. The
COMP voltage and the internal supply rail are
then pulled down. The floating driver is not
subject to this shutdown command.
