MP1470 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
MP1470 Rev. 1.02 www.MonolithicPower.com 10
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resistor to the V
IN
voltage limits the EN input
current to less than 100A.
For example, with 12V connected to Vin,
R
PULLUP
(12V-6.5V) ÷100A =55k
Connecting the EN pin directly to a voltage
source without any pullup resistor requires
limiting the amplitude of the voltage source to
6V to prevent damage to the Zener diode.
EN LOGIC
EN
GND
Zener
6.5V-typ
Figure 4: 6.5V Zener Diode
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) protects the chip
from operating at an insufficient supply voltage.
The MP1470 UVLO comparator monitors the
output voltage of the internal regulator, VCC.
The UVLO rising threshold is about 4.2V while
its falling threshold is consistently 3.85V.
Internal Soft-Start
Soft-start prevents the converter output voltage
from overshooting during startup. When the
chip starts, the internal circuit generates a soft-
start voltage (SS) that ramps up from 0V to
1.2V: When SS falls below the internal
reference (REF), SS overrides REF so that the
error amplifier uses SS as the reference; when
SS exceeds REF, the error amplifier resumes
using REF as its reference. The SS time is
internally set to 1ms.
Over-Current-Protection and Hiccup
The MP1470 has a cycle-by-cycle over-current
limit for when the inductor current peak value
exceeds the set current-limit threshold. First,
when the output voltage drops until FB falls
below the Under-Voltage (UV) threshold
(typically 140mV) to trigger a UV event, the
MP1470 enters hiccup mode to periodically
restart the part. This protection mode is
especially useful when the output is dead-
shorted to ground. This greatly reduces the
average short-circuit current to alleviate thermal
issues and to protect the regulator. The
MP1470 exits 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 temperature exceeds
150°C, it shuts down the whole chip. When the
temperature falls 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, with a
rising threshold of 2.2V and a hysteresis of
150mV. V
IN
regulates the bootstrap capacitor
voltage internally through D1, M1, R4, C4, L1
and C2 (Figure 5). If (V
IN
-V
SW
) exceeds 5V, U2
will regulate M1 to maintain a 5V BST voltage
across C4.
U2
V
IN
U1
5V
D1
M1
R4
C4
SW
L1
C2
V
OUT
Figure 5: Internal Bootstrap Charger Start-Up
and Shutdown Circuit
If both V
IN
and 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 circuits.
Three events can shut down the chip: EN low,
V
IN
low, and thermal shutdown. The shutdown
procedure starts by initially blocking the
signaling path 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.
