LT3645_15 Datasheet, PDF(14/24 Page) Linear Technology – 36V 500mA Step-Down Regulator and 200mA LDO

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LT3645_15 Datasheet, PDF (14/24 Pages) Linear Technology – 36V 500mA Step-Down Regulator and 200mA LDO

◁

LT3645

APPLICATIONS INFORMATION

pull large currents from the output through the SW pin

and the VIN pin. Figure 5 shows a circuit that will run only

when the input voltage is present and that protects against

a shorted or reversed input.

VIN

BOOST

EN/UVLO SW

LT3645

VIN

VCC2

OUT2

EN2

FB2

BACKUP

NPG

GND

3645 F05

Figure 5. Diode D4 Prevents a Shorted Input from Discharging a

Backup Battery Tied to the Output; It Also Protects the Circuit from

a Reversed Input. The LT3645 Runs Only When the Input Is Present

Frequency Compensation (Buck)

The LT3645 uses current mode control to regulate the

loop. This simpliï¬es loop compensation. In particular, the

LT3645 does not require the ESR of the output capacitor

for stability, allowing the use of ceramic capacitors to

achieve low output ripple and small circuit size. A low

ESR output capacitor will typically provide for a greater

margin of circuit stability than an otherwise equivalent

capacitor with higher ESR, although the higher ESR will

tend to provide a faster loop response. Figure 6 shows an

equivalent circuit for the LT3645 control loop.

0.8V

RC 1M

gm = 100Î¼A/V

G = 1A/V

RC = 150k

CC = 60pF

CPL

OUT

COUT

CERAMIC

ESR

ELECTROLYTIC

3645 F06

Figure 6. Model for Loop Response

The error ampliï¬er (gm) is a transconductance type with

ï¬nite output impedance. The power section, consisting

of the modulator, power switch, and inductor, is modeled

as a transconductance ampliï¬er (G) generating an output

current proportional to the voltage at the VC node. Note

that the output capacitor integrates this current, and that

the capacitor on the VC node (CC) integrates the error

ampliï¬er output current, resulting in two poles in the

loop. RC provides a zero. With the recommended output

capacitor, the loop crossover occurs above the RCCC zero.

This simple model works well as long as the value of the

inductor is not too high and the loop crossover frequency

is much lower than the switching frequency. With a larger

ceramic capacitor that will have lower ESR, crossover may

be lower and a phase lead capacitor connected across

R1 in the feedback divider may improve the transient

response. Large electrolytic capacitors may have an ESR

3645f

▷