MP28265EL-LF-P Datasheet MP28265EL-LF-Z, MP28265EL-LF-P, MIC26903-ZAYJL-TR | P10-P12

MP28265 – 5A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP28265 Rev. 0.92 www.MonolithicPower.com 10

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Vin

EN/Sync

Vcc

Vout

CLK

5us

Foldback

1.1MHz

External CLK

0.625*Vout_set

2ms1ms

CC_Rising

Vout_set

Figure 4—Startup Sequence Using External

Sync Clock Signal

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The MP28265 UVLO comparator

monitors the output voltage of the internal

regulator, VCC. The UVLO rising threshold is

about 4.0V while its falling threshold is a

consistent 3.2V.

Internal 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 0V to 1.2V. 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 SS time is internally fixed to

4ms.

Over-Current-Protection and Hiccup

The MP28265 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 MP28265 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 MP28265 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 2.2V with a hysteresis of

150mV. The bootstrap capacitor voltage is

regulated internally by VIN through D1, M3, C4,

L1 and C2 (Figure 5). If (VIN-VSW) is more than

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

voltage across C4.

Figure 5—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

command

MP28265 – 5A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor (see Typical Application

on page 1). Choose R1 to be around 40.2k for

optimal transient response. R2 is then given by:

OUT

−

The T-type network is highly recommended when

Vo is low, as Figure 6 shows.

VOUT

Figure 6— T-type Network

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1—Resistor Selection for Common

Output Voltages

OUT

(V) R1 (kΩ) R2 (kΩ) Rt (kΩ)

1.05 4.99(1%) 16.5(1%) 24.9(1%)

1.2 4.99(1%) 10.2(1%) 24.9(1%)

1.5 4.99(1%) 5.76(1%) 24.9(1%)

1.8 4.99(1%) 4.02(1%) 24.9(1%)

2.5 40.2 (1%) 19.1(1%) 0

3.3 40.2(1%) 13(1%) 0

5 40.2 (1%) 7.68(1%) 0

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

of at least 25% percent higher than the maximum

load current is recommended for most

applications. For highest efficiency, the inductor

DC resistance should be less than 15m. For

most designs, the inductance value can be

derived from the following equation.

OSCLIN

OUTINOUT

fIV

)VV(V

×Δ×

−×

Where I

is the inductor ripple current.

Choose inductor ripple current to be

approximately 30% if the maximum load current,

5A. The maximum inductor peak current is:

LOAD)MAX(L

Under light load conditions below 100mA, larger

inductance is recommended for improved

efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low ESR

capacitors for the best performance. Ceramic

capacitors with X5R or X7R dielectrics are highly

recommended because of their low ESR and

small temperature coefficients. For most

applications, a 22µF capacitor is sufficient.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input capacitor

can be estimated by:

⎟

⎠

⎞

⎜

⎝

⎛

−×=

OUT

LOAD1C

The worse case condition occurs at V

IN = 2VOUT,

where:

LOAD

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic

capacitor, i.e. 0.1F, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

⎟

⎠

⎞

⎜

⎝

⎛

−××

=Δ

OUT

LOAD

1Cf

MP28265 – 5A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

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Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended. Low ESR capacitors are

preferred to keep the output voltage ripple low.

The output voltage ripple can be estimated by:

⎟

⎠

⎞

⎜

⎝

⎛

××

+×

⎟

⎠

⎞

⎜

⎝

⎛

−×

=Δ

2Cf8

ESR

OUT

Where L is the inductor value and R

ESR is the

equivalent series resistance (ESR) value of the

output capacitor.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance. The output

voltage ripple is mainly caused by the

capacitance. For simplification, the output

voltage ripple can be estimated by:

⎟

⎠

⎞

⎜

⎝

⎛

−×

×××

OUT

2CLf8

V

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

ESR

OUT

V ×

⎟

⎠

⎞

⎜

⎝

⎛

−×

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP28265 can be optimized for a wide range of

capacitance and ESR values.

PCB Layout

PCB layout is very important to achieve stable

operation. Please follow these guidelines and

take Figure 7 for references.

1) Keep the connection of input ground and

GND pin as short and wide as possible.

2) Keep the connection of input capacitor and

IN pin as short and wide as possible.

3) Ensure all feedback connections are short

and direct. Place the feedback resistors

and compensation components as close to

the chip as possible.

4) Route SW away from sensitive analog

areas such as FB.

5) Connect IN, SW, and especially GND

respectively to a large copper area to cool

the chip to improve thermal performance

and long-term reliability.

6) Adding RC snubber circuit from IN pin to

SW pin can reduce SW spikes.

Top Layer

Bottom Layer

Figure 7—PCB Layout

P1-P3 P4-P6 P7-P9 P10-P12 P13-P14

MP28265EL-LF-P

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Switching Voltage Regulators 5A, 21V, 1.1MHz Sync Step-down Converter

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