LD8526

08/31/2018

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD8526-DS-00 August 2018

Application Information

Operation Overview

The LD8526 is a secondary side synchronous rectification

driver IC for CCM and DCM operation. The LD8526 not

only has excellent dead time control function for safety in

load transient, but also only needs very low operation

current in green mode. In addition, LD8526 can generate

its own supply voltage through low output voltage or high

side rectification applications to charge battery. Hence,

LD8526 is suitable for PD application.

LDO Charge Function

LD8526 has two LDO (low drop-out regulator), which are

used to provide VCC power. When LD8526 is applied in

low side, VIN is connected to DC output and HVR is

connected to SR MOSFET drain pin. Once the primary

side power on and DC output is below 4.4V, VCC will be

charged by HVR via LDO and VCC will keep at 5.8V at

heavy load. As DC output exceeds 4.4V, VCC will be

replaced by VIN to charge via LDO. When VIN increases,

VCC also rises until 5.2V.

When LD8526 is applied in high side, HVR is connected to

SR MOSFET drain pin, too. However VIN is floating. VCC

can only be charge by HVR via the LDO. And in heavy

load, the VCC is always maintained at 5.8V.

No matter LD8526 is applied in high side or low side, a

capacitor must exist from VCC to GND to store the energy.

Under Voltage Lockout (UVLO) and Enable

Function

An UVLO comparator is implemented in it to detect the

voltage on the VCC pin. It will assure the supply voltage

enough to turn on the LD8526 controllers and further to

drive the synchronous rectifier. As shown in Fig. 15, the

UVLO(ON) and UVLO(OFF) are 3V and 2.8V respectively.

To enable synchronous rectifier, the following conditions

must be met:

1.

VCC > UVLO(ON)

2.

Exit green mode

3.

DET > VDET_ON

With these restrictions, synchronous rectifier always

operates in stable condition. Therefore, some unstable

transient, which are like turn-on, turn-off, output short,

surge, ESD

…etc., will not make synchronous rectifier

unsafe when it works.

UVLO(ON)

UVLO(OFF)

VCC

DR

V

t

t

DET

t

VDET_ON

Fig. 15

Green Mode Operation

For improving the efficiency in light load conditions,

LD8526 stops SR MOSFET driving function to reduce

operation current, as shown in Fig. 16.

DET pin detects system behavior, and the internal logic

circuit sets DRV to high or low. Hence, LD8526 can

enter/exit green mode normally, even under dynamic load.