ADP1147AR-3.3 Ver la hoja de datos (PDF) - Analog Devices

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ADP1147AR-3.3

High Efficiency Step-Down Switching Regulator Controllers

Analog Devices 

ADP1147-3.3/ADP1147-5

Losses are encountered in all elements of the circuit, but the

four major sources for the circuit shown in Figure 1 are:

1. The ADP1147 dc bias current.

2. The MOSFET gate charge current.

3. The I2 × R losses.

4. The voltage drop of the Schottky diode.

1. The ADP1147’s dc bias current is the amount of current that

flows into VIN of the device minus the gate charge current.

With VIN = 10 volts, the dc supply current to the device is

typically 160 µA for a no load condition, and increases pro-

portionally with load to a constant of 1.6 mA in the continu-

ous mode of operation. Losses due to dc bias currents increase

as the input voltage VIN is increased. At VIN = 10 volts the dc

bias losses are usually less than 1% with a load current

greater than 30 mA. When very low load currents are

encountered the dc bias current becomes the primary point

of loss.

2. The MOSFET gate charge current is due to the switching of

the power MOSFET’s gate capacitance. As the MOSFET’s

gate is switched from a low to a high and back to a low again,

charge impulses dQ travel from VIN to ground. The current

out of VIN is equal to dQ/dt and is usually much greater than

the dc supply current. When the device is operating in the

continuous mode the I gate charge is = f (QP). Typically a

P-channel power MOSFET with an RDS on of 135 mΩ will

have a gate charge of 40 nC. With a 100 kHz, switching

frequency in the continuous mode, the I gate charge would

equate to 4 mA or about a 2%–3% loss with a VIN of 10 volts.

It should be noted that gate charge losses increase with

switching frequency or input voltage. A design requiring the

highest efficiency can be obtained by using more moderate

switching frequencies.

3. I2 × R loss is a result of the combined dc circuit resistance

and the output load current. The primary contributors to

circuit dc resistance are the MOSFET, the Inductor and

RSENSE. In the continuous mode of operation the average

output current is switched between the MOSFET and the

Schottky diode and a continuous current flows through the

inductor and RSENSE. Therefore the RDS(ON) of the MOSFET

is multiplied by the on portion of the duty cycle. The result is

then combined with the resistance of the Inductor and

RSENSE. The following equations and example show how to

approximate the I2 × R losses of a circuit.

RDS(ON) × (Duty Cycle) + RINDUCTOR + RSENSE = R

ILOAD2 × R = PLOSS

VOUT × ILOAD = POUT

PLOSS/POUT × 100 = % I2 × RLOSS.

With the duty cycle = 0.5, RINDUCTOR = 0.15, RSENSE = 0.05

and ILOAD = 0.5 A. The result would be a 3% I2R loss. The

effects of I2R losses causes the efficiency to fall off at higher

output currents.

4. At high current loads the Schottky diode can be a substantial

point of power loss. The diode efficiency is further reduced

by the use of high input voltages. To calculate the diode loss,

the load current should be multiplied by the duty cycle of the

diode times the forward voltage drop of the diode.

ILOAD × % duty cycle × VDROP = Diode Loss

Figure 6 indicates the distribution of losses versus load cur-

rent in a typical ADP1147 switching regulator circuit. With

medium current loads the gate charge current is responsible

for a substantial amount of efficiency loss. At lower loads the

gate charge losses become large in comparison to the load,

and result in unacceptable efficiency levels. When low load

currents are encountered the ADP1147 employs a power

savings mode to reduce the effects of the gate loss. In the

power savings mode of operation the dc supply current is the

major source of loss and becomes a greater percentage as the

output current decreases.

Losses at higher loads are primarily due to I2R and the

Schottky diode. All other variables such as capacitor ESR

dissipation, MOSFET switching, and inductor core losses

typically contribute less than 2% additional loss.

Circuit Design Example

In using the design example below assumptions are as follows:

VIN = 5 Volts

VOUT = 3.3 Volts

VDIODE drop (VD) = 0.4 Volts

IMAX OUT = 1 Amp

Max switching frequency (f) = 100 kHz.

The values for RSENSE, CT and L can be calculated based on the

above assumptions.

RSENSE = 100 mV/1 Amp = 100 mΩ.

tOFF time = (1/100 kHz) × [1 – (3.7/5.4)] = 3.15 µs.

CT = 3.15 µs /(1.3 × 10 4) = 242 pF.

L = 5.1 × 10 5 × 0.1 Ω × 242 pF × 3.3 V = 41 µH.

If we further assume:

1. The data is specified at +25°C.

2. MOSFET max power dissipation (PP) is limited to 250 mW.

3. MOSFET thermal resistance is 50°C/W.

4. The normalized RDS(ON) vs. temperature approximation (δP)

is 0.007/°C.

This results in 250 mW × 50°C per watt = 12.5°C of MOSFET

heat rise. If the ambient temperature TA is 50°C, a junction

temperature of 12.5°C +50°C, TA = 62.5°C. δP = 0.007 ×

(62.5°C –25°C) = 0.2625

We can now determine the required RDS(ON) for the MOSFET:

RDS(ON) = 5(0.25)/3.3 (1)2 (1.2625) = 300 mΩ

The above requirements can be met with the use of a P-channel

IRF7204 or an Si9430.

When VOUT is short circuited the power dissipation of the

Schottky diode is at worst case and the dissipation can rise

greatly. The following equation can be used to determine the

power dissipation:

PD = ISC(AVG) × VDIODE Drop

A 100 mΩ RSENSE resistor will yield an ISC(AVG) of 1 A. With a

forward diode drop of 0.4 volts a 400 milliwatt diode power

dissipation results.

The rms current rating needed for CIN will be at least 0.5 A over

the temperature range.

–10–

REV. 0

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