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LT3645

3645f

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.

Frequency Compensation (Buck)

The LT3645 uses current mode control to regulate the

loop. This simpliﬁes 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.

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

Figure 6. Model for Loop Response

The error ampliﬁer (gm) is a transconductance type with

ﬁnite output impedance. The power section, consisting

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

as a transconductance ampliﬁer (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

ampliﬁer 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

3645 F05

VIN

GND

VCC2

NPG

OUT2

VIN

EN/UVLO

EN2

FB2

BACKUP

BOOST

LT3645

gm = 100μA/V

G = 1A/V

RC = 150k

CC = 60pF

OUT

COUT

CERAMIC

0.8V

CPL

ELECTROLYTIC

ESR

3645 F06

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