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LTC7821

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 19a shows a typical LTC7821 output stage setup

while 19b shows the equivalent circuit. Note that M3 and

M4 form the buck converter switches, taking its power

from MID, with its voltage given by:

VMID =

VIN

– IOUT •

VOUT

VIN

•

⎛

⎝⎜

⎞

⎠⎟

• ZMID

where h = efficiency of the buck converter.

A good conservative number to use for h is 0.9.

The above equation gives us the average MID voltage

and does not include the AC ripple on it. The CMID ripple

voltage is given by (see CFLY and CMID Selection section):

VCMID _RIPPLE =

IOUT • tON

2CMID

Therefore the maximum deviation of MID voltage from

VIN/2 is given by:

ΔVMID _IDEAL =

VIN

– VMID –

IOUT • tON

2CMID

UsetheaboveequationasaguidelinetosettheHYS_PRGM

voltage.

Design Example

A 48V to 5V, 25A operating at 500kHz application is used

as an example. Since the output current is high, DCR

sensing is used to regulate the current loop.

For 500kHz operation, a 68k resistor is connected from

FREQ to ground.

Since VIN = 48V, Infineon BSC027N06LS5 is chosen for

M1. This is an 60V MOSFET. For M2 to M4, 30V MOSFETs

are sufficient for this application. For M2 and M3, Infineon

BSC032N04LSareselectedandanInfineonBSC014N04LSI

is selected for M4.

M1, M3 DUTY CYCLE =

= 0.208

TOFF = (1– 0.208)• 2µs = 1.58µs

With inductor current ripple initially scaled to 40% of

output current, ΔIL = 0.4 • 25 = 10A,

Hence, L =

VOUT • TOFF

ΔIL

5 •1.58 •10 – 6

= 0.79µH

An inductance of 0.9µH is selected (Coilcraft SER2011-

901L) instead and this give ΔIL = 8.8A. The RMS current

through the inductor is:

IRMS _L = I2OUT+

ΔI2L

⎛

⎝

⎜

⎞

⎠

⎟

= 25

2 + 0.4

= 25.2A

From the manufacturer data sheet, the rise in temperature

oftheinductoris15°C.However,thisrisedoesnotaccount

for the conduction of heat from the MOSFETs through the

PCB that increases the overall inductor’s temperature.

Depending on how the board is being laid out and how

much air flow is applied, the net increase in temperature

could be higher. For this design example, assume the net

temperature rise to be 50°C.

The typical DCR of the inductor is typically 1.2mΩ with a

maximum of 1.34 mΩ and with a 50°C rise in temperature

and assuming the temperature coefficient of the DCR at

0.4%/°C, the maximum DCR is:

RDCΔ = 15°C = 1.34 • 1+

0.4 • 50

100

⎛

⎝⎜

⎞

⎠⎟

•10 –3

= 1.61m Ω

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