Figure 12. SS/TRK clampler operation

6.4

Error amplifier

The error amplifier (EA) provides the error signal to be compared with the sawtooth to perform the pulse width

modulation. Its non-inverting input is internally connected to a 0.8 V voltage reference, while its inverting input

(FB) and output (COMP) are externally available for feedback and frequency compensation. In this device, the

error amplifier is a voltage mode operational amplifier so with high DC gain and low output impedance.

The uncompensated error amplifier characteristics are the following:

Table 8. Error amplifier characteristic

Characteristic

Value

Non-inverting input voltage reference voltage

800 mV

Low frequency gain (A0)

95 dB

GBWP

6.5 MHz

Output voltage swing

0.3 to 3.3 V

Source / sink current capability

20 / 45 mA

In continuous conduction working mode (CCM), the transfer function of the power section has two poles due to

the LC filter and one zero due to the ESR of the output capacitor. Different topologies of compensation networks

can be used depending on the ESR value of the output capacitor.

If the zero introduced by the output capacitor helps to compensate the double pole of the LC filter, a type II

compensation network can be used. Otherwise, a type III compensation network must be used (see Section xx

Compensation network for details on the compensation network design).

Anyway, the methodology to compensate the loop is to introduce zeros to obtain a safe phase margin.

6.5

Embedded gate driver supply

The embedded gate driver power supply is the 7.5 V typ. LDO regulator output available on the VCC pin for the

external compensation. Connect an mlcc capacitor value between 470 nF and 2.2 µF for proper operation.

The typical regulator dropout voltage is 250 mV @ VIN = 6 V, IVCC = 20 mA so the VCC voltage may drop at

input voltage lower than 7.75 V. Be aware that the VCC drop at low input voltage may increase “not logic level

MOSFET” RDSON, affecting nominal programmed OCP threshold and increasing switching / conduction losses.

For operation compatible with low input voltage, select “logic level MOSFET” with guaranteed maximum VTH = 4.5

V.

L3751

Error amplifier

DS13841 - Rev 1

page 16/41