NB621 – 8A, 25V, 600kHz SYNCHRONOUS STEP-DOWN CONVERTER WITH INTEGRATED LDO

NB621 Rev. 0.9

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11

12/10/2013

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regulator is in full regulation. When VIN is lower

than 5.0 V, the output of the regulator decreases.

Since this internal regulator provides the bias

current for the bottom gate driver that requires

significant amount of current depending upon the

external MOSFET selection, a 1 µF ceramic

capacitor for decoupling purpose is required.

Enable/Sync Control

The NB621 has a dedicated Enable/Sync control

pin (EN/SYNC). By pulling it high or low, the IC

can be enabled and disabled by EN. Tie EN to

VIN for automatic start up. To disable the part,

EN must be pulled low for at least 5µs.

The NB621 can be synchronized to external

clock range from 300 kHz up to 2 MHz through

the EN/SYNC pin. The internal clock rising edge

is synchronized to the external clock rising edge.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The NB621 UVLO comparator

monitors the output voltage of the internal

regulator, VCC. The UVLO rising threshold is

about 4.0 V while its falling threshold is a

consistent 3.2 V.

Soft-Start

The soft-start is implemented to prevent the

converter output voltage from overshooting

during startup. When the chip starts, the internal

circuitry generates a soft-start voltage (SS)

ramping up from 0 V to 1.2 V. When it is lower

than the internal reference (REF), SS overrides

REF so the error amplifier uses SS as the

reference. When SS is higher than REF, REF

regains control. The soft start time (

ss

calculated by:

s s

s s

6

1 . 2

C

T

1 0

1 0

−

⋅

=

⋅

Over-Current-Protection and Hiccup

The NB621 has cycle-by-cycle over current limit

when the inductor current peak value exceeds

the set current limit threshold. Meanwhile, output

voltage starts to drop until FB is below the Under-

Voltage (UV) threshold, typically 30% below the

reference. Once a UV is triggered, the NB621

enters hiccup mode to periodically restart the part.

This protection mode is especially useful when

the output is dead-short to ground. The average

short circuit current is greatly reduced to alleviate

the thermal issue and to protect the regulator.

The NB621 exits the hiccup mode once the over

current condition is removed.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip

from

operating

at

exceedingly

high

temperatures. When the silicon die temperature

is higher than 150

°C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 140

°C, the chip is

enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is 2.2 V with a hysteresis of

150mV. The bootstrap capacitor voltage is

regulated internally by VIN through D1, M3, C4,

L1 and C2 (Figure 2). If (VIN-VSW) is more than

5 V, U2 will regulate M3 to maintain a 5V BST

voltage across C4.

SW

Figure 2—Internal Bootstrap Charging Circuit

Startup and Shutdown

If both VIN and EN are higher than their

appropriate 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

stable

supply

for

the

remaining

circuitries.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In 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